Template, build system caching and improved loading.

 * Support multiple template and build system directories.
 * Also search in $USERDATADIR/valama/{templates,buildsystems} for templates /
   build systems.
 * Cache available packages (Guanako), locales, templates and build systems
   and provide read only view on them. (There is a bug for maps, related to
   #666728)
 * Provide default (builtin) empty template.
 * Support creating ValamaProject/ProjectFile from data string.