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_sigfrx.hpp

@@ -32,6 +32,7 @@
 #include "max2837.hpp"
 #include "volume.hpp"
 #include "receiver_model.hpp"
+#include "radio_state.hpp"
 
 namespace ui {
 
@@ -50,6 +51,11 @@ class SIGFRXView : public View {
     uint8_t last_channel;
     uint8_t detect_counter = 0;
 
+    RxRadioState radio_state_{
+        1750000 /* bandwidth */,
+        3072000 /* sampling rate */
+    };
+
     const uint16_t sigfrx_marks[18] = {
         10, 8, 0,
         60, 52, 90,