Make pressing Enter confirm project creation/import in the project manager

This makes the project manager more friendly to keyboard usage.

You can now create projects more easily without touching the mouse
by opening the project manager, pressing Ctrl + N, entering a project
name and pressing Enter.

scene/scene_string_names.h

@@ -154,6 +154,7 @@ public:
 	StringName confirmed;
 
 	StringName text_changed;
+	StringName text_submitted;
 	StringName value_changed;
 };