Currently the only way for a uuser to invoke corner-tiling is to
manually set shortcuts for them. This patch adds another option: invoke
the existing shortcuts for edge tiling in a combined manner in quick
succession.

For example, hitting Meta+Left and then Meta+Up within a one-second
period will tile the active window into the top left corner. In practice
you hold down the Meta key and then press Left then Up (or Up and then
Left), and I think it feels very natural. Linux Mint's window manager
has this feature and I always missed it when I left Mint for the KDE
world.

Autotests for existing tiling shortcuts are adjusted to not break, and
additional tests for the new tiling options are added.