Use '~' prefix instead of '%' for random brushes (#362)

Fixes #337.

src/Autoload/Import.gd

@@ -68,7 +68,7 @@ func get_brush_files_from_directory(directory: String): # -> Array
 			# and png one at that nya
 			if !the_directory.current_is_dir() and curr_file.get_extension().to_lower() == "png":
 				# if we are a random element, add
-				if "%" in curr_file:
+				if "~" in curr_file:
 					randomised_files.append(curr_file)
 				else:
 					non_randomised_files.append(curr_file)

src/UI/Dialogs/PreviewDialog.gd

@@ -196,14 +196,14 @@ func add_brush() -> void:
 		dir.list_dir_begin()
 		var curr_file := dir.get_next()
 		while curr_file != "":
-			if curr_file.begins_with("%") and brush_name in curr_file:
+			if curr_file.begins_with("~") and brush_name in curr_file:
 				random_brushes.append(curr_file)
 			curr_file = dir.get_next()
 		dir.list_dir_end()
 
 		var file_ext : String = path.get_file().get_extension()
 		var index : int = random_brushes.size() + 1
-		var file_name = "%" + brush_name + str(index) + "." + file_ext
+		var file_name = "~" + brush_name + str(index) + "." + file_ext
 		var location := "Brushes".plus_file(brush_name).plus_file(file_name)
 		dir.copy(path, Global.directory_module.xdg_data_home.plus_file(location))