Split executable and empty file "types" into seperate "prop" flag

[tmccombs]

But keep the old file types for backwards compatibility.

Fixes #823

This is a work in progress. I haven't added anything to the changelog yet, and the documentation could probably be a little better. I'm also not entirely sure if this is something we want to do.

Note that the new --prop options behave slightly differently than the old file types. In particular:

• If no --type option is given, then empty defaults to just empty files, not files and directories. That makes it more consistent with the way executable works, seems more useful to me, and is simpler to implement, but I could be convinced to change this to default to files and directories.
• The executables property no longer always includes files, so you can search for just executable directories. I'm not really sure when that would be useful, but it seems more consistent and logical to me.

[sharkdp]

Thank you very much for exploring this path. I will have to think a bit more about this. I really like that we currently only have one option instead of two. On the other hand, if the behavior is really inconsistent, we should probably sacrifice simplicity for correctness.

Does this PR itself introduce any breaking behavior, i.e. do you plan to remove -tx?

[tmccombs]

iirc, it doesn't actually remove -tx to preserve backwards compatibility, but it does remove documentation from the help and man page.

[sharkdp]

Closing for now, see #823