[Client] Improve keyboard movement

The movement of a character was not triggered in case the key to move
was just pressed a short time. This issue was caused by the delay timer
that is in place to allow proper movement with two keys. This issue is
now fixed. In case the delay timer did not expire, the movement is
triggered once the key is released.
[Client] Hide questmarkers in case the tile is hidden

Questmarkers of different map levels were visible in a unexpected way.
This is now resolved by hiding the markers in case the tile the marker
belongs to is hidden.