Blender Git Loki

UI: Improvements to interaction with active modifier

This commit includes these improvements to operators and how they
relate to the active modifier:

- Shortcuts performed while there is an active modifier will affect
only that modifier.
- Clicking anywhere on the empty space in a modifier's panel will make
it active.
- Clicking anywhere in the empty properties space will clear the
active.

These changes require some refactoring of object modifier code. First
is splitting up the modifier property invoke callback, which now needs
to be able to get the active modifier separately from the hovered
modifier for the different operators. Second is a change to removing
modifiers, where there is now a separate function to remove a modifier
from an object's list, in order to handle changing the active. Finally
the panel handler neeeds a small tweak so that this "click in panel"
event can be handled afterwards.