The memory group shows memory-usage information: the currently used
memory size, the available physical memory size, and the total
physical memory size.  It can also show the tile-cache size, for
comparison against the other memory stats.  The memory group is
active but contracted by default.

Note that the upper-bound of the meter is the physical memory size,
so the memory usage may be > 100% when GIMP uses the swap.

This is currently implemented for *nix systems with Linux-like
procfs, and Windows.