git-gui: Convert merge dialog to use class system

git-gui: Convert merge dialog to use class system

I've found that the class code makes it a whole lot easier to create
more complex GUI code, especially the dialogs.  So before I make any
major improvements to the merge dialog's interface I'm going to first
switch it to use the class system, so the code is slightly cleaner.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>