dirmodel: Fix crash when incorrect index is returned

This happens when dragging a file into the root of a paper directory.
Add a Filejpegpage class
file: Fix an unused parameter warning
file: Make the Filepage destructor virtual

We really should not have a derived class with a non-virtual destructor
even if it is currently empty.
desktopview: Fix unused var warning
utils: Use const for removeExtension() filename
Allow Filejpeg to have multiple pages/files

A JPEG file of the form "base_p<page>.jpeg" will be consider to be a page
of a larger stack of JPEG files. We identify these when opening the desktop,
grouping all matching JPEGs into stacks through an extremely inefficient
n^2 algorithm which will suffice for small directories.

So far, such files do not stack/unstack correct, and there are many other
problems to resolve / features to implement.