Fix redraw problem after canceling "Canvas Size" command
Change Mask member functions to use gfx::Rect instead of x,y,w,h args
Minor change in undo label from selection commands
Add Ctrl+T to transform the cel content (close #560)
Remove minor comment in gui.xml
Use an internal ImageBuffer inside Mask class
Add setting/preference classes/enums generator

Changes:
* Add a class (Option<T>) to get/set/listen changes to one specific
  preference option
* Add data/pref.xml with the metadata to generate types/classes (not it's
  quite easy to add new preferences)
* Modify the generator to support generation of .h and .cpp files of
  preference types
* Add code to migrate old options to new ones (and functions to delete
  old values)
* Only a couple of options were migrated at the moment, it is a WIP, in the
  future we should completely remove ISettings and direct calls
  to set/get_config_*
Unify all render code in one library

Changes:
* Create render library (move util/render.cpp to render/render.cpp)
* Move app::Zoom class to render::Zoom
* Remove doc::Image::merge() member function
* Add gfx::Clip helper class (to clip dst/src rectangles before a blit)
* Move doc::composite_image() to render::composite_image()
* Remove doc::Sprite::render()
* Replace Sprite::getPixel() with render::get_sprite_pixel()
* Remove doc::layer_render() function
* Convert DitheringMethod to a enum class
* Add AppRender to configure a render::Render with the app configuration
* Move checked background preferences as document-specific configuration
* Add doc::Sprite::layer() and palette() member functions
* Add doc::Layer::cel() member function
* Add doc::Palette::entry() member function()
* Add doc::frame_t type
* Move create_palette_from_rgb/convert_pixel_format to render library
* ExportSpriteSheet doesn't need a temporary image now that we can specify
  the source rectangle in the render routine
Replace doc::FrameNumber with doc::frame_t
Replace doc::LayerImage::getCel() with doc::Layer::cel()