Fix computing screen geometry on multiple HiDPI monitors

When using setups with multiple HiDPI displays, Yakuake did not show up
on all the other monitors except for the top-left one, due to the fact
that the screen geometry is not properly computed, so
getDesktopGeometry() returns QRect(0,0 0x0). The QScreen::geometry()
method appears to behave "weird" on HiDPI displays: the top left corner
is given in physical pixels, while the width,height are in device
independent pixels. For example, if using two 4K displays, one on top
and one on bottom, the QScreen::geometry() will return
QRect(0,2160 1920x1080) for the bottom one.