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 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...

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.

When the screen is composited, use a nicer area selection mecanism. Draw
a fullscreen transparent window which is darker than the normal view.
When the user draws a rectangle with the mouse, show the "normal" view
in this rectangle: the area which will be grabbed is normal, the rest
of the screen is darker.

A "traditionnal" rubber banding ala Thunar or Xfdesktop could be nicer,
this has to be investigated.

Add two new options: the title of the capture is now customizable and
is used to generate the name of the saved captures. It is now also
possible to append the date and the time of capturing to the name of
the saved file (TODO: check if "horodating" exists in English).

Fix a leak in the panel plugin.

In some cases, the WM/X server does not update the active window
quickly enough. When running the application and taking a screenshot
of the active window this can create a corrupted screenshot. Adding
a one second delay in that case fixes this issue.

Now that the idle func no longer creates the dialog, it shouldn't
be needed anymore. This needs some testing on a low computer.