Switch to client side decorations

Related to xfce/libxfce4ui#14 (closed).

This uses a standardized slim header bar for all windows, except for dialogs where a header bar with extra widgets already exists, like the open and save as dialogs (when the user has not unset "gtk-dialogs-use-header").

This also removes the close button from dialogs when it is the only one, like in the preferences dialog or the error dialog (shown below).

This should overall give something more homogeneous and quite pleasant I think, escaping pretty much those huge header bars.

Before/After (I had to cut a selection in CSD, because xfce4-screenshooter see the windows as independent):