Add headless regression tests for Prop Editor functionality

- Implement tests for PropFile, including preset conversion, file dictionary conversion, round-trip serialization, version rejection, default values for missing keys, flattening, sanitization, and save/load functionality.
- Create tests for PropEditor, covering window title formatting, mode button text, safe saving, and recent file list management.
- Add tests for PropTheme, validating fallback behavior, invalid input handling, and default values for primitives and UI tokens.
- Introduce tests for ThumbnailCache, ensuring proper key formatting, PNG handling, and behavior with missing files.

scripts/thumbnails/thumbnail_cache.gd

@@ -39,6 +39,14 @@ func prop_png(id: String) -> String:
 	return PROP_DIR + "/" + id + "_v" + str(PROP_VERSION) + ".png"
 
 
+func prop_file_key(path: String) -> String:
+	return "%s_%d" % [path.get_file().get_basename(), FileAccess.get_modified_time(path)]
+
+
+func prop_file_png(path: String) -> String:
+	return PROP_DIR + "/" + prop_file_key(path) + ".png"
+
+
 func load_png(png_path: String) -> Texture2D:
 	if _memory.has(png_path):
 		return _memory[png_path] as Texture2D