Use @rpath so frameworks can be loaded from the app bundle

@executable_path is the originally-executed program, whereas @loader_path is the program that caused the load to occur (e.g. a Quick Look generator). @rpath can be changed at link time — a QL generator can specify a value that points to the enclosing app bundle’s Frameworks directory.
Implement ‘-=’ for target files

Presently only for “string keys” like compiler and linker flags.
Add Quick Look generator

Closes #1062