[GTK] Add hex color format check (#112)
[GTK] Add hex color format check (#112)

GLib throws an error when the background_color string is not in valid
hex format. This fix adds a format check using a regex like we did in
cocoa.py.

Also fix Python 3.x error: "Gtk.CssProvider.load_from_data TypeError:
Item 0: Must be number, not str"
r0x0r/pywebview,r0x0r/pywebview,r0x0r/pywebview,r0x0r/pywebview,shivaprsdv/pywebview,shivaprsdv/pywebview,r0x0r/pywebview,shivaprsdv/pywebview,shivaprsdv/pywebview