Support fractional frame rates
This change adds fractional frame rate support to Krita. Trying not to complicate the user interface, fractional rates spin out of the frame rate selection box below one (the initially proposed sequence was { "3/2", "5/4", "4/5", "2/3", "1/2", "1/3", "1/4", "1/5", "1/10" }. The frame rate was initially the bare int in many places. The specialized KisFrameRate structure is now used. It has the registration to work as signal parameter and also with QVariant.
The change extends both export dialog and story board including some places where div and mod operations were previously used with the integer rate; now this is implemented to work with fractions as well.
As such it is rather big change. It can be splinted into export dialog and story board changes but having fractional rates in one place and only integer rates in another makes the application inconsistent. Simply replacing int with double would may create bugs where the rounding inevitably occurs.
I tested the changed version, it exports with fractional rates no problem, and story board also displays and plays fractional rates correctly. With the proposed change, fractional frame rates should become available everywhere across the Krita.