Add fall height ruler to editor

[Wuzzy2]

I suggest to make the fall height ruler available in the editor, too. It should use the same default shortcut key as in the game (Tab), for consistency.

[SimonN]

Yes, the game's ruler would be handy in the editor.

Technically, it's more complicated than it looks: The game renders both the graphical level and a physics map (of solidity, steel, trigger areas), but the editor only renders the graphics, not the physics. I should find an efficient way to compute this physics map in the editor whenever they're required for the ruler.

[SimonN]

Workarounds in 0.9.17:

The fall height is 126 pixels alive, 127 pixels dead. We can stack two large 64x64 square pieces, e.g. geoo's steel, as a poor man's ruler. This height of 128 is a deadly fall, but a builder's brick or plaformer's brick is 2 pixels high and enough to make a fall of 128 survivable.

There would be a guideline tile in images/guideline/maxfall, but, because of bug #344, none of the pieces from images/guideline/ can be inserted from the terrain browser in 0.9.17.