Updated FindMagnum.cmake from Magnum repository.
TgaImageConverter: export symbols for use as other plugin dependency.
Removed superfluous namespaces.

Not sure how to handle everything properly (e.g. namespaces and naming
for implementation classes, tests...), will update coding style for
plugins accordingly later.
Properly prefix visibility macros.
Adapted to Magnum changes.

Image constructor parameter reordering.
Adapted to Magnum changes.

AbstractFont interface improvements.
Doxygen fixes.
FreeTypeFont: initial test.

Due to GlyphCache the test needs to have active GL context, even if the
code tested doesn't touch anything related to GL. Test for filling glyph
cache (which will touch GL) is not done yet.
HarfBuzzFont: initial test.

Similar to FreeTypeFont. Filling glyph cache is done in FreeTypeFont,
thus no need to test it here. However we still need active GL context
for glyph cache.