Update Tajik translations
gtkwindow: Rename title_icon, etc. to titlebar_icon

"title_box" is used for both a custom header bar and for a titlebar.
Since we want to help differentiate these cases in the code, rename
everything titlebar-internal to use "titlebar_".

https://bugzilla.gnome.org/show_bug.cgi?id=706529
gtkwindow: Have a separate "titlebar" pointer for decorations we create ourselves

This cleans up the code a bit.

https://bugzilla.gnome.org/show_bug.cgi?id=706529
gtkwindow: Split code that detects if we want CSD out

This will get a bit more complex when we enable CSD for custom headerbars

https://bugzilla.gnome.org/show_bug.cgi?id=706529
gtkwindow: Disable CSD if we aren't running under a supported WM

Also, split the checks for CSD out into a separate function.

https://bugzilla.gnome.org/show_bug.cgi?id=706529
gtkwindow: Enable CSD when windows set a titlebar

https://bugzilla.gnome.org/show_bug.cgi?id=706529