Bit of misc clean up on ResourceCache & Font

* Font: Relocate NOM_DEBUG_FONT_SHARING preprocessor define to Font.cpp
* Font: Rename NOM_DEBUG_FONT_SHARING to
NOM_DEBUG_OUTPUT_FONT_FACE_ON_SHARE.
* Font: Swap out the recently removed NOM_DUMP_EXT macro with
NOM_DUMP_VAR (equivalent functionality).

* ResourceCache: Declare find_resource a const method.

* system.hpp: Add public header file 'nomlib/system/resource_handlers.hpp'.
Clean up err handling in engine's init interfaces

* SystemFonts: Drop the log level of the "not initialized" message
from LOG_ERR to LOG_INFO.

* SystemColors: Drop the log level of the "not initialized" message
from LOG_ERR to LOG_INFO.

* PlatformSettings: Warn end-user (developer) of non-initialization
by logging an info level message.

* Add some additional err logging within the nom::init method.
Introduce auto-init of own needs in SystemFonts && PlatformSettings

* Introduce nom::InitHints enumeration & implement logic to handle each
flag within the nom::init_third_party method.
  Note that the default behavior for initialization of third party
libraries remains the same as it was; passing InitHints::DefaultInit --
which was previously always being passed as zero (0) -- initializes all
the SDL extensions.

* SystemFonts: Manage its own third party lib initialization needs;
this allows us to not explicitly call the initializing method(s)
anywhere  within our code-base, and indeed automates the management
of such details by handing the responsibility detail to higher-level
interfaces.

* PlatformSettings: Manage its own third party lib initialization needs;
this allows us to not explicitly call the initializing method(s)
anywhere within our code-base, and indeed automates the management of
such details by handing the responsibility detail to higher-level
interfaces.
Flesh out FontCacheTest::StaticInterfaceAPI unit test.

* FontCacheTest: Rename StaticInterfaceAPI to
GlobalSystemFontsInterface.

* FontCacheTest: Introduce GlobalPlatformSettingsInterface
unit test.