FontCache: Restructuring of init (introduce static helper interfaces)

* SDLApp: Removal of fonts_ class variable & getter method; the global
nom::SystemFonts interface replaces access to it.
* SDLApp: Font enumeration relocated to
PlatformSettings::enumerate_fonts.

* ResourceCache: Explicit clear on resources container upon
destruction.
FontCacheTest: Change ResourceCache type to Font
UIWidget: Introduce font setter for Font raw pointers.

* Font: Introduce DEFAULT_FONT_SIZE (12 pt) const var.
Clean up code a bit; remove phased out IFont code fragments
Bit of clean up on Font & ResourceCache interface

* Font: Introduce raw_ptr & shared_ptr typedefs.
* Font: Remove operator = method overload; ease of future maintenance.
* Font: Add file extension detection for Data Fork Suitcase (.dfont)
type.
* ResourceCache: Introduce erase( key ) method.
FontCacheTest: Introduce StaticInterfaceAPI unit test