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

This code was tested on Xubuntu 14.04 (dual screen) and Windows 8.1 (single screen). Note: I couldn't reproduce the problem on linux, since the window manager would not allow to open a window off the screen. One could decide to not use this code on linux (don't know about mac) but I think a consistent behavior on all OS would be desirable.