CppEditor::OverViewTreeView: Don't depend on ICore::mainWindow()

Change-Id: Ic212807eda4a56f54fd5c8f37c5ab1645b74243f
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
OverViewComboBox: Remove ::sync method, call expandAll() directly

Change-Id: Ia30801e85bffc53434048189150ed2b09a09d7ae
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>
OverviewComboBox: coding style

Change-Id: I69a68419c54ee6d366bca98f47d3ca2e9353766d
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>
OverViewComboBox: Move creation of TreeView into ctor

So that the class is more self-contained.

Change-Id: Ib6f3e5811d4d7d8647a797f451ec1ff8cc003b74
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>