Use the canvas_items stretch mode by default

This is the stretch mode that works best for non-pixel art 2D and 3D
games in general (and even many pixel art games, provided texture
filtering is disabled).

The `canvas_items` stretch mode also ensures that 2D elements are
always scaled to a readable size, even at high resolutions. This is
especially important when targeting mobile platforms. The difference
is immediately noticeable when running a new project in the
experimental editor Android port :)

Since most people are using Godot to develop games (rather than
applications), it makes sense to use a stretch mode that works
best for games by default.

Author: Calinou

main/main.cpp

@@ -2823,7 +2823,7 @@ bool Main::start() {
 			startup_benchmark_file = String();
 		}
 #endif
-		GLOBAL_DEF_BASIC("display/window/stretch/mode", "disabled");
+		GLOBAL_DEF_BASIC("display/window/stretch/mode", "canvas_items");
 		ProjectSettings::get_singleton()->set_custom_property_info("display/window/stretch/mode",
 				PropertyInfo(Variant::STRING,
 						"display/window/stretch/mode",