Correct screen size detection

The XRandR api was used in a such way to get the *DEFAULT* size instead the
current one. This patch create a helper function (detect_display_size()) which
detects the current size of the screen; it  is called both in the
GuiCalibratorX11() constructor and in the redraw() method.

How reproduce the problem:
1) compile xinput for X11 ( autogen.sh --with-gui=x11)
2) change the resolution to a not default one (like xrandr -s 640x480)
3) start xinput_calibrator
4) the result is a window size equal to the size of the *DEFAULT* resolution of the screen, and not the current one
5) expected result: a window size equal to the size of the *CURRENT* resolution of the screen