Add filesystem watch functionality to enable auto update of diagrams

[bkryza]

For some use cases it would be useful to be able to run clang-uml with a --watch option, that would generate diagrams, and then subscribe for filesystem events. In case any of the files matching any of the glob patterns in any diagram changes, it would automatically regenerate such diagram.

This could be done either by using non-portable OS specific mechanisms such as inotify, or by looping forever on the list of translation units from compile_commands.json and comparing timestamps (this would be portable through std::filesystem).

The main problem is that this will only detect changes in the main translation unit cpp file, not in any of its headers. For this we could extend diagram generation to return a list of all included headers, and cache it so on the next loop iteration we can watch those files too...