Move XSLT files into a Qt resource

[Thiago Macieira]

This patch series moves the .xslt files and the xslt/ subdir in the
installation into a Qt resource. That means it's in a read-only
section of memory instead of on disk. Benefits are that we don't need
to install and manage the files, nor play tricks to find them at
runtime. Drawbacks are that they are now always loaded into memory
(albeit in a read-only, thus sharable and discardable page) and
they'll most likely require decompression before use.

If this is a direction we're comfortable with, I can look into doing
the same for other types of file resources.