Places the simple style editor in a frame with an Apply button.

This commit introduces a new dialog in SIF and the associated method in
UIFactory. It provides a way to build a dialog with OK, Apply and
Cancel buttons. The dialog contains a single UIPanel. The Apply
behaviour is controled through an ActionListener.

I've intentionally chosen not to use this panel for the advanced style editor. I've started working on a new advanced editor and I don't want to do the work twice. Moreover, the current advanced style editor already has an Apply button.