* src/aboutdialog.cc (SelectableLabel): New fancy experiment to make labels
both selectable _and_ focusable, so that they end up in the Tab chain.

* src/aboutdialog.cc src/imagebutton.cc src/prefdialog.cc: Don't show all
children of custom "composite" widgets in the constructor.  It doesn't make
much sense since we call show_all() later on anyway.  And making these
"composite" widgets really independent components would require overriding
both show_all_vfunc() and hide_all_vfunc(), which is currently not worth
the effort.