Pressing Ctrl when using the Wang tool should
also be able to make it _not_ fill whole
tiles when held, and fill whole tiles when
not held.

Still need to make this toggle-able from
a toolbar or something similar.

Add action to the Terrain Tool's toolbar which toggles
this mode.

Holding Ctrl/Cmd still temporarily switches to the mode
that's not currently selected.

Users can add their own shortcut key from
the preferences menu if they want.

This has been removed on master. I added
to it to make the fill full tiles icon appear
but now that doesn't seem to be necessary.

This came in through the merge with master. It's
not there in master and not added by commits in this
branch so it can be removed.

We can just read out the checked state of mToggleFillFullTiles.

Also corrected name of icon variable.