Stash radio settings per app (#1151)

* add a little Stash utility.
* add radio state RAII helper.
* first part of radio_state changes
* add radio_state_ to rest of apps
* fix freq_step and format
* fix unused ui_sigfrx: corrected sample rate, added back freq setting

firmware/application/apps/ui_playlist.cpp

@@ -202,9 +202,6 @@ void PlaylistView::start() {
 
     if (reader) {
         button_play.set_bitmap(&bitmap_stop);
-
-        baseband::set_sample_rate(sample_rate * 8);
-
         if (now_delay) {  // this `if` is because, if the delay is 0, it will sleep forever
             chThdSleepMilliseconds(now_delay);
         }