TextEditor: get CompletionAssistProvider from editor.

And not from the global object pool. This way, the editors that have
different highlighters for various language dialects, or editors that
support multiple languages in a single editor, can decide themselves on
what CompletionAssistProvider to provide.

Change-Id: Ieebc4a8e7b3de6470fdb8103035aa3b8b2ba6598
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>
QtSupport: Fix 'Copy Project to writable Location?' dialog handling

 * fixes Keep and Open dialog option
 * does nothing if user closes the dialog window

Change-Id: I3e84621e8314450c4dca2408ae2738ebcd1b33e6
Reviewed-by: Daniel Teske <daniel.teske@digia.com>