fix startup: register GtkSourceView GType for GtkBuilder

Removing gobject.type_register(GtkSource.View) broke startup: the
call was load-bearing, it forced registration of the GtkSourceView
GType so GtkBuilder could resolve the type in damaris3.glade
("Invalid object type 'GtkSourceView'"). Restore the registration
by referencing gtksourceview.View at import time, which is the
same effect without the PyGTK compat API.

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

src/damaris/gui/DamarisGUI.py

@@ -35,6 +35,10 @@ from gi.repository import Gdk as gdk
 
 from gi.repository import GtkSource as gtksourceview
 
+# referencing the class registers the GtkSourceView GType, which GtkBuilder
+# needs to resolve the type when loading damaris3.glade
+_ = gtksourceview.View
+
 from gi.repository import Pango as pango
 
 # array math