SDL_helpers: Introduce pixel, alpha_pixel variant methods
Image: Improve init method (improves rendering of 32-bit RGBA TTF)
TrueTypeFont: Introduce font getter method
Remove ariali.ttf from examples/app/Resources
MessageBox: Bit of clean up on the class
Label: Introduce copy assignment, font getter, texture getter

* Bit of general clean up work, such as renaming render_font_ to
texture_
examples/CMakeLists: Re-enable compiling of all examples