[Themes] Remove our copies of Eclipse themes and reference originals

- We don't have to maintain our copies of the theme files
- Add our specific css files to plugin.xml as stylesheet extensions
- Keep the "System" Eclipse theme as it no longer ships with Eclipse
- Don't show the "High Contrast" theme in Preferences combo

com.archimatetool.editor.themes/META-INF/MANIFEST.MF

@@ -9,4 +9,5 @@ Bundle-Vendor: Archi
 Eclipse-BundleShape: dir
 Automatic-Module-Name: com.archimatetool.editor.themes
 Require-Bundle: org.eclipse.e4.ui.css.swt.theme,
- org.eclipse.ui
+ org.eclipse.ui,
+ org.eclipse.ui.themes

com.archimatetool.editor.themes/plugin.properties

@@ -18,7 +18,7 @@ PALETTE_ARROW_BUTTON_FOREGROUND=Palette arrow button foreground
 VIEW_BACKGROUND=View background
 VISUALISER_BACKGROUND=Visualiser background
 
-#New theme element definitions
+#New theme element definitions (needed for System theme)
 DARK_BACKGROUND=Dark Background Color
 DARK_FOREGROUND=Dark Foreground Color
 INACTIVE_UNSELECTED_TABS_COLOR_START=Inactive, unselected part color begin

com.archimatetool.editor.themes/plugin.xml

@@ -13,65 +13,36 @@
 
 	<extension
        point="org.eclipse.e4.ui.css.swt.theme">
-		<theme
-			basestylesheeturi="themes/eclipse/css/e4_classic.css"
-			id="org.eclipse.e4.ui.css.theme.e4_classic"
-			label="%theme.classic">
-		</theme>
-		<theme
-			basestylesheeturi="themes/eclipse/css/e4_default_gtk.css"
-			id="org.eclipse.e4.ui.css.theme.e4_default"
-			label="%theme.light"
-			os="linux">
-		</theme>
-		<theme
-			basestylesheeturi="themes/eclipse/css/e4_default_mac.css"
-			id="org.eclipse.e4.ui.css.theme.e4_default"
-			label="%theme.light"
-			os="macosx">
-		</theme>
-        <theme
-            basestylesheeturi="themes/eclipse/css/e4_default_win.css"
-            id="org.eclipse.e4.ui.css.theme.e4_default"
-            label="%theme.light"
-            os="win32">
-        </theme>
-        <theme
-            basestylesheeturi="themes/eclipse/css/e4-dark_win.css"
-            id="org.eclipse.e4.ui.css.theme.e4_dark"
-            label="%theme.dark"
-            os="win32">
-        </theme>
-        <theme
-            basestylesheeturi="themes/eclipse/css/e4-dark_linux.css"
-            id="org.eclipse.e4.ui.css.theme.e4_dark"
-            label="%theme.dark"
-            os="linux">
-        </theme>
-        <theme
-            basestylesheeturi="themes/eclipse/css/e4-dark_mac1013.css"
-            id="org.eclipse.e4.ui.css.theme.e4_dark"
-            label="%theme.dark"
-            os="macosx"
-            os_version="10.11,10.12,10.13">
-        </theme>
-        <theme
-            basestylesheeturi="themes/eclipse/css/e4-dark_mac.css"
-            id="org.eclipse.e4.ui.css.theme.e4_dark"
-            label="%theme.dark"
-            os="macosx">
-        </theme>
-        <theme
-            basestylesheeturi="themes/eclipse/css/e4_system.css"
-            id="org.eclipse.e4.ui.css.theme.e4_system"
-            label="%theme.system">
-        </theme>
+    <stylesheet
+          uri="themes/eclipse/css/dark/archi-dark-styling.css">
+       <themeid
+             refid="org.eclipse.e4.ui.css.theme.e4_dark">
+       </themeid>
+    </stylesheet>
+    <stylesheet
+          uri="themes/eclipse/css/light/archi-light-styling.css">
+       <themeid
+             refid="org.eclipse.e4.ui.css.theme.e4_default">
+       </themeid>
+    </stylesheet>
+    <stylesheet
+          uri="themes/archi/css/common/archi-globalstyle.css">
+       <themeid
+             refid="org.eclipse.e4.ui.css.theme.e4_classic">
+       </themeid>
+    </stylesheet>
+    <theme
+          basestylesheeturi="themes/eclipse/css/system/e4_system.css"
+          id="org.eclipse.e4.ui.css.theme.e4_system"
+          label="%theme.system">
+    </theme>
 	</extension>
  <extension
        point="org.eclipse.ui.themes">
     <themeElementCategory
           id="com.archimatetool.editor.themes"
-          label="%themeCategory.archi">
+          label="%themeCategory.archi"
+          parentId="org.eclipse.ui.themes.CssTheme">
     </themeElementCategory>
     <colorDefinition
           categoryId="com.archimatetool.editor.themes"