drawwindow: no longer unfullscreen on Escape

Escape is now only for exiting the current mode, since we have buttons for
exiting fullscreen now which are always available.
drawwindow: switch filename and app name order

* Update the format string for titlebars with filenames
* Also mark the strings for translation, just in case

I've noticed that most of the programs on my desktop suffix filenames with
the app name, not vice versa.  This has the effect of putting the more
user-relevant information, namely the file basename, at the front of the
titlebar string.  That allows crowded window lists in panels to display
more of the user-relevant information despite their truncation of titlebar
strings.
drawwindow: re-enable DnD; use python-gi dialect

* Re-enable dropping of colours and files from other apps
* Use python-gi names, and remove gtk2compat stuff
drawwindow: bump copyright date
help strings: remove "hover and press key" info

This help text is misleading now that GTK no longer supports it.

Towards https://gna.org/bugs/?21751 (but not a fix...)
application: use python-gi; Turkish l18n

* Use python-gi names, and remove gtk2compat stuff
* Remove dead GTK2 code
* Fix Turkish l18n issue (case for ıIiİ, see URL in comment)
* Explicitly use new-style classes everywhere