Add button in midi settings to refresh midi devices

Deletes and creates a new RT midi instance allowing for midi devices to be plugged in without restarting

src/engine/engine.h

@@ -1077,6 +1077,9 @@ class DivEngine {
     // rescan audio devices
     void rescanAudioDevices();
 
+    /** rescan midi devices */
+    void rescanMidiDevices();
+
     // set the console mode.
     void setConsoleMode(bool enable);