Core: moved DnD from event filtering to MainWindow overloading.
Overall: migrate to index-based windowing.

Some things still don't work:
* Docks aren't properly associated with tabs.
* Pintab can't pin anything but in the first window.
* TSM doesn't work (or works for first window only).
TSM: adapt to multiwindow.
Core: no use for MainWindow in LocalSocketHandler.
Core: get rid of {Get,Set}ReallyMainWindow.
Core: fixed dock widget visibility signature.
SB2: update dock hook.
Core: properly close windows on unload.
Pogooglue: fixed googling for URLs.
LHTR: white background.
Core: add new tab menu to LeechCraft menu.
Core: TabManager::GetWidgetCount().
Core: properly handle window closes.
SB2: properly handle new windows.
Core: handle single-tab readditions across windows.
Core: tabs moves across windows.
Core: menu for tab moves.
Core: action for new window creation.
Core: humans count from 1.
Core: removed debugging output.