Improve automatic mode switching on page load

Previously, Normal mode was _always_ entered on location change (page load)
(unless the page is blacklisted; then Ignore mode is entered instead). There
were to reasons for this: Handling the blacklist, and auto-exiting hints mode
(because hints left over from another page does not make sense in a new page).
However, this caused a few problems:

- If you used the `zF` command while a page was loading, the markers would
  disappear when the page fired the "location change" event.
- There was an ugly special case for Find mode, making sure not to enter Normal
  mode if currently in Find mode (see commit fc7e61e).
- Sometimes, you can enter Hints mode on a newly loaded page just before the
  page fires the "location change" event, making hints annoyingly disappear.

This commit changes two things:

- Hints mode is automatically exited (by returning to Normal mode) on the
  'pagehide' event instead.
- The current mode is only changed on location change _if needed._ That is, to
  auto-exit Ignore mode after navigating away from a blacklisted page, or
  auto-enter Ignore mode on blacklisted pages.

extension/lib/events-frame.coffee

@@ -95,6 +95,7 @@ class FrameEventManager
 
       if target == @vim.content.document
         messageManager.send('frameCanReceiveEvents', false)
+        @vim.enterMode('normal') if @vim.mode =='hints'
 
       # If the target isn’t the topmost document, it means that a frame has
       # changed: It could have been removed or its `src` attribute could have

extension/lib/vim.coffee

@@ -33,6 +33,7 @@ ChromeWindow = Ci.nsIDOMChromeWindow
 
 class Vim
   constructor: (browser, @_parent) ->
+    @mode = undefined
     @focusType = 'none'
     @_setBrowser(browser, {addListeners: false})
     @_storage = {}
@@ -128,11 +129,12 @@ class Vim
     return suppress
 
   _onLocationChange: (url) ->
-    unless @mode == 'ignore' and @_storage.ignore.type == 'explicit'
-      if @_isBlacklisted(url)
+    switch
+      when @_isBlacklisted(url)
         @enterMode('ignore', {type: 'blacklist'})
-      else
-        @enterMode('normal') unless @mode == 'find'
+      when (@mode == 'ignore' and @_storage.ignore.type == 'blacklist') or
+           not @mode
+        @enterMode('normal')
     @_parent.emit('locationChange', {vim: this, location: new @window.URL(url)})
 
   _call: (method, data = {}, extraArgs...) ->