Trade: use Corrade::Containers::Array in AbstractImageConverter.
Trade: reworked AbstractImporter plugin interface.

Implementation is moved into private virtual `do*()` functions and the
public interface does additional checks aroung them to simplify plugin
development. Opening files is by default done by the base
implementation, which then calls function for opening raw data with file
contents.

Added test for file opening, bumped plugin interface version to 0.3.
Y# Please enter the commit message for your changes. Lines starting
Trade: reworked AbstractImageConverter plugin interface.

Implementation is moved into private virtual `do*()` functions and
public interface does additional sanity checks around them. Exporting to
file is by default done in base implementation, which takes exported
data and then saves them to the file.

Added unit test for file export, bumped plugin interface version to 0.2.
Text: reworked AbstractFont plugin interface.

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.