test/io/file_io: added is_open() tests.
test/gfx/display: corrected context version.
resource_manager: initial package control and resource handling.

resource_package:
	initial definition;¹
	added interface.

resource_manager:
	added package collection and package addition/removal;
	changed resource map value to type-value pair;
	added has_handler();
	added load_resource() and unload_resource().

1: Ideally, the current structure will only be used for filesystem
access (i.e., unpacked package), with directory watching to react
to file changes. The manifest will probably go away at some point.
Built packages will map resources only to a position in the package
stream.
test/general/headers: updated.
test/resource/manager: new test.