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_level.hpp

@@ -38,6 +38,7 @@
 #include "string_format.hpp"
 #include "file.hpp"
 #include "app_settings.hpp"
+#include "radio_state.hpp"
 
 namespace ui {
 
@@ -53,6 +54,7 @@ class LevelView : public View {
    private:
     NavigationView& nav_;
 
+    RxRadioState radio_state_{};
     app_settings::SettingsManager settings_{
         "rx_level", app_settings::Mode::RX};