Key event handler to cancel LM placement

[ivanjonas]

When targeting mode is enabled to select a location for the LM, it may be difficult or impossible to return to the button that cancels targeting. In that case, pressing Esc should cancel targeting mode.

(This issue previously specified an event handler to also confirm the placement of the LM. This is no longer necessary because clicking the target that is currently being hovered is equivalent to clicking the drop zone. )