to avoid confusion, increment $VERSION of all Tk::Color* modules
TkTest.pm: retry_update
entry.t: be graceful on slow/loaded WMs

Using the new retry_update in a couple of places. The old
TODO_xscrollcommand_problem hack is gone. Also, some (but not all!)
unnecessary ->update calls in selection-handling code is also gone, to
avoid concurrency problems.
create_placeholder_widget: now in TkTest.pm

Also, it seems that progbar.t also is need of the placeholder.
fbox.t: destroy only if window changesd visibility before

A fix to the occasional
    window ".fbox" was deleted before its visibility changed
problem.
travis-ci: test using different window managers

Also different -dev package availability (with or without libpng and
libjpeg) is tested.
add kwin to test matrix
add xfwm4 to matrix, remove metacity and fluxbox