Make methods static and add a instance() method for Signals/Slots.

Remove ProjectExplorerPlugin::taskHub() method and use the new
ones instead.

Change-Id: Ifae24ff19579fc524cbd61bddc826095c443adfa
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
Reviewed-by: hjk <hjk121@nokiamail.com>