Implementation is moved into `do*()` private virtual functions, public
interface is doing additional sanity checks around them. Opening files
is by default done in base implementation, which loads the files into
memory and then calls function for opening raw data.
Added interface for opening multi-file fonts from raw data, default
implementation just calls single-file implementation.
Function for creating glyph cache converts the text from UTF-8 to UTF-32
and then calls the implementation, removing the burden from
reimplementing this in each plugin.
Added unit test for file and single data opening, bumped plugin version
to 0.2.
Font and text layouting is now abstracted out from its implementation.
Font class is renamed to FreeTypeFont and all the work can be done
through AbstractFont and AbstractLayouter interface.