search-path: filter out duplicates

Note: this does not yet take care of filtering dups
when directly combining two search-paths.
fix meterbridge: load button image only once.
consolidate session_dirs: use Searchpath instead of custom code
add Session::StateProtector

temp. disable save during batch updates, save once at
the end.
use state Session::StateProtector
use a hash-table to cache gui properties
add RAII DisplaySuspender
suspend route redisplay for (rec-en, solo, mute and monitor) batch changes