GUI: prepare for more UX improvements

- add "temporary instrument" to DivEngine
- prepare for two things:
  - instrument preview on file picker (at least built-in one)
  - instrument selector for banks (#364)

src/engine/engine.h

@@ -302,6 +302,7 @@ class DivEngine {
 
   public:
     DivSong song;
+    DivInstrument* tempIns;
     DivSystem sysOfChan[DIV_MAX_CHANS];
     int dispatchOfChan[DIV_MAX_CHANS];
     int dispatchChanOfChan[DIV_MAX_CHANS];
@@ -525,6 +526,9 @@ class DivEngine {
     // if the returned vector is empty then there was an error.
     std::vector<DivInstrument*> instrumentFromFile(const char* path);
 
+    // load temporary instrument
+    void loadTempIns(DivInstrument* which);
+
     // delete instrument
     void delInstrument(int index);
 
@@ -798,6 +802,7 @@ class DivEngine {
       metroAmp(0.0f),
       metroVol(1.0f),
       totalProcessed(0),
+      tempIns(NULL),
       oscBuf{NULL,NULL},
       oscSize(1),
       oscReadPos(0),