Suggestion/feature request: use dlopen() for GTK+

[ell1e]

The README suggests using GTK+ dialogs requires to link GTK+, which automatically makes a build unusable on Linux systems where it is installed. For greater versatility of the resulting binaries, wouldn't it be possible to require GTK+3 only at build time by using dlopen() and function pointers instead (and otherwise only its headers but no link) to access it? I think that'd be a great addition to allow more universal binaries. It would also allow the library to report some sort of recoverable error if neither GTK+ nor zenity are available, such that the calling program could possibly fall back to other alternatives if desired.