As spotted by Gauvain Pocentek, the "while" loop in the previous code
was not the best thing in the world as it lead to 100% CPU usage during
the selection. Instead, we use a filter function. This needs testing and
it would be great to share some code with the composited version.

This allows us to avoid a lot of artifacts which occured in some
weird and random cases whithout using ugly hacks. Moreover, it
makes sense as the active window is not really defined 0 seconds
after the dialog is closed (the WM needs some time to focus a new
window).

As spotted by Mike Massonnet, this was simply the separator of the
fullscreen window...

This should fix saving of preferences when the rc file does not exist
yet. Note: always test things with a clean configuration...

Thanks to Mike for noticing this function.

The text was copied to the primary clipboard and was thus only
accessible via middle click.

As Mike explained to me, this is useless. The clipboard manager will
automatically take the image when the application is closed.

This caused xfce4-screenshooter -r to be unusable as a command for a
keyboard shortcut, as the keyboard was already grabbed by the thingy
which launches applications from shortcuts. Escape is still not usable
in that case, but that's normal as we don't grab the keyboard...

Instead we launch that action directly, supposing that if the user
specified an action, he expects it to be used and not to be asked
again what he wants to do.

Do not use the frame extensions to get the screenshot of a window,
but use its shape instead. This allows us to have a transparent
background under rounded corners instead of the window that is
under the screenshoted window.

Fix build with full debugging and plug a leak.