[gnome-builder/wip/gtk4-port] add gtk4 porting notes

[Christian Hergert <chergert src gnome org>]

commit 0b99ec73fb065f59b55983da55d2aa95d7216b45
Author: Christian Hergert <chergert redhat com>
Date:   Fri Sep 24 13:20:09 2021 -0700

    add gtk4 porting notes

 TODO-GTK-4.md | 12 ++++++++++++
 1 file changed, 12 insertions(+)
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diff --git a/TODO-GTK-4.md b/TODO-GTK-4.md
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+# GTK 4 Notes
+
+This document is meant to help keep track of things that we will need to come back to as part of the GTK 4 
port.
+
+## IdeTree
+
+ * DnD drop targets need work, as they aren't being handled yet. This will be easier with plugins to test it.
+ * The cell renderer uses cairo drawing. Instead we should use icons for the Git states so we get GDK 
texture caching.
+
+## IdeRunManager
+
+ * We don't have a short manager for GTK 4 yet. We will want an accessory object for IdeApplication or 
similar. Alternatively, the workspace should monitor the IdeRunManager for changes and add/remove shortcuts.