GUI - don't erase undo buffer history when setting text

Sonic Pi's editor calls the underlying text widget's method setText() when loading a file.

This is implemented as follows:

```
// Set the given text.
void QsciScintilla::setText(const QString &text)
{
    bool ro = ensureRW();

    SendScintilla(SCI_SETTEXT, ScintillaBytesConstData(textAsBytes(text)));
    SendScintilla(SCI_EMPTYUNDOBUFFER);

    setReadOnly(ro);
}
```

Note that it explicitly clears the undo buffer. We override this behaviour
in this commit. There's also no need to fiddle with the "read only" state.

Undo now works as expected after loading a file (you can now undo and redo
the load file action).

Author: samaaron

app/gui/qt/widgets/sonicpiscintilla.cpp

@@ -680,3 +680,7 @@ void SonicPiScintilla::showAutoCompletion(bool val) {
     setAutoCompletionThreshold(-1);
   }
 }
+
+void SonicPiScintilla::setText(const QString &text) {
+  SendScintilla(SCI_SETTEXT, ScintillaBytesConstData(textAsBytes(text)));
+}

app/gui/qt/widgets/sonicpiscintilla.h

@@ -82,6 +82,7 @@ class SonicPiScintilla : public QsciScintilla
     void sp_cut();
 
     void showAutoCompletion(bool val);
+    void setText(const QString &text);
  private:
     void addKeyBinding(QSettings &qs, int cmd, int key);
     void addOtherKeyBinding(QSettings &qs, int cmd, int key);