Add support for SVG and Pixmap based previews to the Unavail shape

Further, this commit includes a minor refactoring of the draw function
which now stores the questionmark in the pimpl.

Previews are loaded immediately during loading time. This is
reasonably simple to refactor to lazy loading at a later point in
time if needed.

Finally, a loadFile function was added (and saveFile refactored to
use it). Function used to load file contents of preview file.