Check package versions with pkg-config.

 * Check package versions with pkg-config and display detailed information in
   template selector.
 * Support removal of package choices (only individual packages at the
   moment).
    - Clear choice if empty.
 * Use TreeMultiMap for ValamaProject.packages list. That makes
   ValamaProject.package_list obsolete so remove it.
 * Escape all variable text in markup labels and use StringBuilder.