Revert NOM_DISABLE_GFX_LOG_TRACE macro

* The macro got lost somewhere in all the merging as of late. Oh well!
VBoxLayoutTest: Fix the test failures with layout spacing
examples/fonts: Fix crash when font is NULL
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