SSH: first click after fullscreen-on-connect leaves the mouse button held, every move selects text #256

Symptom

On an entry set to open fullscreen on connect, the first click after an SSH session connects leaves the terminal in a drag: moving the pointer selects whole blocks of lines (screenshot shows the selection with nothing pressed). The same can happen when the fullscreen gesture is a key press: the key stays held on the remote.

Cause

Browsers only enter fullscreen from a user gesture, so client.html arms a one-shot capture listener for the next mousedown or keydown after CONNECTED and calls requestFullscreen() from it. That listener does not stop the event, so Guacamole.Mouse (or Guacamole.Keyboard) forwards the press to guacd as usual. The fullscreen transition then resizes and re-lays out the display element, and the matching mouseup (or keyup) never reaches the element: Guacamole.Mouse listens for releases on the element only. guacd is left with the button held, and the terminal treats every subsequent pointer move as a drag selection.

Fix

The gesture press is the browser's, not the remote's: stop its propagation so the display's handlers never see it. On fullscreenchange into fullscreen, release everything explicitly anyway: keyboard.reset() and a mouse state with no buttons at the last pointer position, so a release lost for any other reason during the transition cannot leave the session stuck.

Folded into #255.