Solves app and category buttons focus issues

The buttons were grabbing the focus out of the search entry on hover,
inducing strange behaviour.

Fixes #23
Correction of previous commit - clear when clicked

Now category buttons only clear text when clicked, not when hovered.
It seems more natural, because a now the mouse position is irrelevant
when trying to type on the search entry.
Merge branch 'button-focus' of https://github.com/hordepfo/mintmenu into hordepfo-button-focus

Conflicts:
	usr/lib/linuxmint/mintMenu/plugins/applications.py