gui.windowing: remove UTILITY hint from SubWindow

The hint caused Gnome >= 3.30 (and possible older versions as well) to
spawn the icon editor window under the brush editor window when opened
from the latter (not from the menu). While this may be an upstream bug
in the Gnome WM, removing the hint does not appear to have any negative
impact when using other WM's, including in Windows 10.

Closes #1110

gui/windowing.py

@@ -70,12 +70,6 @@ def __init__(self, app, key_input=False):
         # Only hide when the close button is pressed if running as a subwindow
         if app:
             self.connect('delete-event', lambda w, e: self.hide_on_delete())
-        # Mark subwindows as utility windows: many X11 WMs handle this sanely
-        # This has caused issues with OSX and X11.app under GTK2/PyGTK in the
-        # past. OSX builds no longer use X11.app, so this should no longer
-        # need special-casing. Testers: use if not sys.platform == 'darwin':
-        # if needed, and please submit a patch. https://gna.org/bugs/?15838
-        self.set_type_hint(Gdk.WindowTypeHint.UTILITY)
         # Win32 and some Linux DEs are responsive to the following: keeps the
         # window above the main window in fullscreen.
         if app: