Valama: Extend project format: defines, custom vapis

 * Incompatible with previous format.
Valama: Support custom vapis (project file).

 * Support package checks to enable defines based on package version and/or
   use custom vapis based on version (or independent of it).
 * Allow to disable dependency tracking for packages/vapis (project format
   change).
 * Guanako:
    - Add static method to get list of vapi/deps directories: get_vapi_dirs
    - Add static method to look for vapi/deps file:
      get_vapi_path, get_deps_path
    - Add get_vapis method to get used vapis (analogous to get_source_files).
 * Drop some macros: PIXMAP_DIR, VALA_VAPI_DIR, VALA_DATA_DIR
Don't save custom_vapis back to project file if enabled by extracheck.
Don't try to guess already enabled defines.