Taskwindow: Allow for Keyboard shortcuts in actions

Allow to trigger TaskHandler actions by keyboard shortcut. Simplify
the code while doing so.

As a side effect the default handler is now listed in the context menu,
which makes it more discoverable.

Change-Id: I41183e5a5ef77be57360f29f289e327cadfaa7d1
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>