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_sstvtx.hpp

@@ -34,6 +34,7 @@
 #include "file.hpp"
 #include "bmp.hpp"
 #include "app_settings.hpp"
+#include "radio_state.hpp"
 
 using namespace sstv;
 
@@ -56,6 +57,7 @@ class SSTVTXView : public View {
 
    private:
     NavigationView& nav_;
+    TxRadioState radio_state_{};
     app_settings::SettingsManager settings_{
         "tx_sstv", app_settings::Mode::TX};