Rename Text::Style::Underlined to Text::Style::Underline
Add override keyword where appropriate
TrueTypeFontTest: Add glyph cache test
TrueTypeFont: Clean up fixme && todo notes

* TrueTypeFont: Rename internal class var kerning_ to use_kerning_.
Introduce BitmapFontTest
Remove Resources/fonts dir

The 'Resources/fonts' directory -- a relic of nom::PlatformSettings and
friends -- has been removed, and FontCacheTest has been refactored to
use the available resources from 'Resources/tests/graphics' using the
nom::SearchPath interface.

* FontCacheTest: Rename output executable from font_cache to
FontCacheTest.