Unify resource names

We have no use of file extensions to access the \BlueLaTeX resources, it
is much cleaner to have uniform resource names, also for generated pdf
files and all other pdf stuffs. Same applies for the zip archive that
can be downloaded.

This work prepares for the API extension required by #18.
Add API to retrieve number of pages

Once the paper is compiled, one can access the number of pages that was
produced in the compiled version.

closes #18
Rename `File` utilities

This class has become much more general purpose than it used to be.
Rename the utility class to make it much clearer when used.
Merge remote-tracking branch 'upstream/master' into wip-web-client