[Suggestion] Integrate qwebdavlib as git submodule

[Olf0]

Consider integrating mhaller's qwebdavlib as a git submodule, despite the project being dead (e.g. for Qt 4 & 5, only). CepiPerez seems to have simply copied the files available at that time (which is pretty close to its final state by mhaller). All forks seem to carry only minor changes, though different ones, but mostly not conflicting ones. Mmmh, I do not want to get dragged deeply into FileCase's maintenance aside of infrastructure aspects, but this calls for assembling these changes in a(nother) fork.