Fixes for Vala 0.18 and warning cleanup. Dropped set_filter_name 0.14 workaround.

- [x] Fix Vala 0.18 compilation error (issue #17)
- [x] Fix Vala 0.18 compiler warnings (issues #17, #18, #19)
- [x] Remove Vala 0.14 workarounds (drops Vala 0.12 support) (issue #15)
- [x] Merge after @Falc refactor