Change default refresh rate to 60Hz

This changes the default refresh rate (which KWin uses if it cannot get the rate from the screen) from 50Hz to 60Hz.
There are two reasons for this:
- When plugging out the last active screen, even without calling xrandr the X server changes the screen geometry to some built-in minimum (320x200), with, of course, no information about the refresh rate. As a result, KWin assumes 50Hz and restarts the compositor to accommodate for this change, only to change it again soon thereafter when there's a screen again.
- I know of nobody using a 50Hz screen. Most people use 60Hz, some use 120Hz, but 50Hz seems pretty rare to me.

REVIEW: 110454