Introduce File::user_home_path && File::system_path methods

* osx/PlatformSettings: Use ::user_home_path && ::system_path
methods for system & library path creation.

* windows/PlatformSettings: Use ::user_home_path && ::system_path
methods for system path creation.
PlatformSettings: Introduce user fonts paths

* osx/PlatformSettings: Add the DroidSans font face from the generated
user fonts path.

* osx/PlatformSettings: Add the LiberationSans-Regular font face from
the generated user fonts path & remove addition of the
LiberationSans-Regular font face from the engine's resources bundle
location.

* osx/PlatformSettings: Add the LiberationSerif-Regular font face from
the generated user fonts path & remove addition of the
LiberationSerif-Regular font face from the engine's resources bundle
location.
Introduce NOM_DEBUG_OUTPUT_LAYOUT_DATA preprocessor define

* Toggle the debug dump of layout data within UIWidget::set_layout &&
UIBoxLayout.
Fix VBoxLayoutTest errs

* The unit tests now execute without problem on both OS X & Windows. This
is the result of several of the recent commits made -- in no particular
order: auto-init logic, invalid context bug fix in RenderWindow, and so
forth.