Elevate getContent() and setContent() to IFilePath.

Requires an implementation of setContent() for ZipPath, which wasn't too hard.
However, it'll now also require some testing and refactoring up above to make
the entire system work correctly.
zippath: Make ZipArchives writeable.
memory: Stop using AbstractFilePath.
readonly: Stop using AbstractFilePath.
zippath: Stop using AbstractFilePath.
filepath: Stop using AbstractFilePath.
Remove AbstractFilePath.

Composition is better than inheritance.