Display the build date in the editor and when starting the engine

This can be used to quickly see how recent a development build is,
without having to look up the commit date manually.
When juggling around with various builds (e.g. for benchmarking),
this can also be used to ensure that you're actually running the
binary you intended to run.

The date stored is the date of the Git commit that is built, not
the current date at the time of building the binary. This ensures
binaries can remain reproducible.

The version timestamp can be accessed using the `timestamp` key
of the `Engine.get_version_info()` return value.

core/config/engine.cpp

@@ -114,6 +114,8 @@ Dictionary Engine::get_version_info() const {
 	String hash = String(VERSION_HASH);
 	dict["hash"] = hash.is_empty() ? String("unknown") : hash;
 
+	dict["timestamp"] = VERSION_TIMESTAMP;
+
 	String stringver = String(dict["major"]) + "." + String(dict["minor"]);
 	if ((int)dict["patch"] != 0) {
 		stringver += "." + String(dict["patch"]);