Blender Git Loki

Change interaction of pie menus slightly.

In drag style pie menus clicking is ignored now. This is done because
if we want to ensure that pie menus will not respawn, we had to not
destroy and keep the pie menu invisble while the button is pressed.

This opened a can of worms where pie items that would spawn popups could
steal the focus from the invisible pies and then the former pies would
be stuck in an invisible state, stealing all input unless user did a
right click to cancel the pie.

Invisible state is still kept for cancelling and strictly for that only.

This still allows nested layouts but only in click style, while hold
style is used for faster interaction. As a plus, it should also be
possible to tweak slider properties now, though menu will close after
that.