In the spirit of **search everywhere**, one way to significantly speed up
interaction with modifiers is to allow searching to add them.
This patch adds a context sensitive search menu operator to the relevant
tabs of the properties editor. Another benefit is that this allows adding
modifiers, etc, when you're scrolled down past the "Add Modifier" button.
Two notes about the implemention:
1. I had to add a check for an empty identifier to the operator search code
in order not to display the column separators in the search menu.
2. This changes the default "invoke" action for the relevant operators to
the search menu. This seems fine to me, but maybe there is a better
way to do it.