Added built-in performance logging into Krita

Brief instructions:

1) Add 'enablePerfLog=true' into your kritarc
2) Start/restart Krita to enable it
3) Choose your problematic image/preset
4) Do as many strokes as you can. Of different speed and size
5) You will get './log/' folder where the preset and the log are stored
6) Run krita/sdk/plot_strokes_statistics to visualize the data

Still TODO:
1) Measure how much time is spent on openGL loading and updating part
   of pipeline.
2) Write a detailed manual for users about how to generate this log
3) Dump some hardware info, like CPU, GPU, dirver version and so on.

CCMAIL:kimageshop@kde.org