Skip the method PluginsTab::performApply until after activation

If this method (or part of it) is run before the tab has been activated
then the default configuration passed as parameter will be modified and
incorrect values will be introduced.

Remove attributes in PDE launch configurations instead of explicitly
setting them to their default values

The attributes in  question are:
- DESELECTED_WORKSPACE_BUNDLES --> []
- USE_CUSTOM_FEATURES --> false
- SHOW_SELECTED_ONLY --> false
- INCLUDE_OPTIONAL --> true
- AUTOMATIC_ADD --> true

The reason to do this is that 2 launch configurations are considered
different if one of them explicitly sets an attribute to its default
value (because it then has 1 more attribute)

This commit fixes a regression introduced in
98a5865

Fixes #1250

org.eclipse.pde.doc.user/forceQualifierUpdate.txt

@@ -1,3 +1,3 @@
 Update to IPDELauncherConstants
 https://github.com/eclipse-pde/eclipse.pde/issues/1250
-https://github.com/eclipse-platform/eclipse.platform.releng.aggregator/issues/2024
+https://github.com/eclipse-platform/eclipse.platform.releng.aggregator/issues/2024