Fix issues caused by live modifying Modifiers

It mostly makes sense now, though at some point I'll make probably
make all persistent modifiers read-only.
Add a nice hover effect for the mod list
Create phantom modifier at the end

This means there will be a delegate at the end, and allow the user
to click on it and type, which will 'promote' it to a volatilte
modifier.

The behavior is very GMail like at the moment.
Fix bug when inserting rows to Attribute

Bloody off-by-one errors.
Fix up layout of modifier delegate

That Positioner should really have been a Layout Manager the whole
time.
Enable selection by mouse

To make the thing feel more realistic.

Note: redo the selection manually to follow GMail's one more
closely.
Move ModifiersListView to a better spot

It's now on the right, and encased in a Flickable, so it will now
scroll instead of clipping off screen.