ui: Fix new chat window opens only to the right #85

On Windows a new chat window always got opened right to the main window
even if this position was off the screen.

Every time the user opens a new chat window the following is happening:

 - a dictionary (name -> geometry) of all screens is generated

 - the screen name of the screen on which the main window is located is
   being fetched

 - a decision based on screen geometry is made to determine where to
   open the new window