Make font dpi fully configurable by the user

Adopts startkde.cmake to accept dpi values different from 96 and 120.

Adopts the UI in kcontrol to make dpi values from 1 to 1000 possible.
The spinbox uses steps of 24 dpi because the common dpi values 72,
96 and 120 are all multiples of 24. As some fonts are optimized for
these resolutions, this could make fonts look better.

FIXED-IN: 4.7
REVIEW: 101410
GUI:
DIGEST: