Fix WM_CLASS for correct icon matching in docks and taskbars

[newman2x]

When mintlocale-im is launched, its WM_CLASS is reported as "im.py"/"Im.py"
because the entry point script is named im.py. This causes dock and taskbar
applications to fail to match the window to its .desktop entry — "im" is too
generic and matches nothing.

Changes:

• Added GLib to existing gi.repository import in im.py
• Added GLib.set_prgname("mintlocale-im") to correctly set the application name
• Added Gdk.set_program_class("Mintlocale-im") to correctly set the WM_CLASS
• Added StartupWMClass=mintlocale-im to mintlocale-im.desktop as a fallback
  for dock/taskbar matching

Tested on Linux Mint 22.3 XFCE with xfce4-docklike-plugin.