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.

doc/classes/Engine.xml

@@ -180,6 +180,7 @@
 				- [code]status[/code] - Status (such as "beta", "rc1", "rc2", "stable", etc.) as a String;
 				- [code]build[/code] - Build name (e.g. "custom_build") as a String;
 				- [code]hash[/code] - Full Git commit hash as a String;
+				- [code]timestamp[/code] - Holds the Git commit date UNIX timestamp in seconds as an int, or [code]0[/code] if unavailable;
 				- [code]string[/code] - [code]major[/code], [code]minor[/code], [code]patch[/code], [code]status[/code], and [code]build[/code] in a single String.
 				The [code]hex[/code] value is encoded as follows, from left to right: one byte for the major, one byte for the minor, one byte for the patch version. For example, "3.1.12" would be [code]0x03010C[/code].
 				[b]Note:[/b] The [code]hex[/code] value is still an [int] internally, and printing it will give you its decimal representation, which is not particularly meaningful. Use hexadecimal literals for quick version comparisons from code:
@@ -261,7 +262,7 @@
 				func _enter_tree():
 				    # Depending on when the node is added to the tree,
 				    # prints either "true" or "false".
-				    print(Engine.is_in_physics_frame()) 
+				    print(Engine.is_in_physics_frame())
 
 				func _process(delta):
 				    print(Engine.is_in_physics_frame()) # Prints false