Tidy up mainframe (creating a new dialog class).
Fix OptionalJSpinner having an excessive border.

Fixes issue CLIENT-66
Fixes issue CLIENT-2

Change-Id: I450fe4a6060988843caad0b22a1ac2ffad9e4c28
Depends-On: If58397d37b89c8fe6e099c47fb4d26fdc4589b49
Reviewed-on: http://gerrit.dmdirc.com/1630
Automatic-Compile: DMDirc Local Commits <dmdirc@googlemail.com>
Reviewed-by: Chris Smith <chris@dmdirc.com>