Fix UISettingsImpl/UIPenSettingsImpl::removeObserver() functions
Add pixel-perfect drawing mode for freehand tools (issue 315)

* Added new app::tools::IntertwineAsPixelPerfect intertwiner
* Added app::tools::FreehandAlgorithm enum
* Added app::tools::WellKnownIntertwiners constants
Update copyright year in config.h and README.md