* src/controller.{cc,h}: New files, implementing class Controller
and class ControlItem.  ControlItem maintains an arbitrary number
of activation sources, and controls the sensitivity state of an
arbitrary number of widgets too.  Controller centralizes all of
the ControlItems and provides create() functions for all pure
control widgets in regexxer.  Also, there's ControlGroup which can
be used to set the sensitivity of several widgets at once.

All this stuff became necessary due to the proposed addition of
menus.  Getting the sensitivity of all widgets right was already
quite complex.  After the integration of menus the old code would
have reached a critical mass.

* src/imagebutton.{cc,h}: New files, containing ImageButton and
ImageLabelButton that were previously in mainwindow.cc.  We need
them in controller.cc now too.

* src/Makefile.am (regexxer_SOURCES): Add new files.

* src/mainwindow.{cc,h}: Integrate the new Controller concept and
get rid of the obsolete cruft.  We're now below 1000 lines again.