Improve performance and stability of gmic

o add benchmarks for fast method converting Krita layers to gmic
o optimize converting Krita RGBA layers to gmic layer and back to Krita layers

KisGmicBenchmarks::testConvertToGmic(): 6,673 msecs
KisGmicBenchmarks::testConvertFromGmic(): 7,149 msecs

KisGmicBenchmarks::testConvertToGmicFast(): 59 msecs (113-times faster)
KisGmicBenchmarks::testConvertFromGmicFast(): 154 msecs (46-times faster)

Slow method with LCMS2 is used when the colorspace is different from RGBA.
Optimization is implemented as color-transformation that only scale
RGBA colorspace depth to float.

Now is the performance in pair with g'mic for gimp plug-in

o fix crash when closing gmic widget
o add more unit tests converting gmic-krita conversions
o add simple feedback that gmic filtering is done:
- window title contains information about running time of gmic filter