Don't allow DX7 type for OSC2 just yet

follow up to SynthstromAudible#3004

This restriction was intentional as the UI code (like the operator
sidebar) assumes only OSC1 is used as a DX7 oscillator.

We might want to improve the UI later on, but for now keep the
restriction

src/deluge/gui/menu_item/osc/type.h

@@ -34,7 +34,7 @@ class Type final : public Selection, public FormattedTitle {
 	Type(l10n::String name, l10n::String title_format_str) : Selection(name), FormattedTitle(title_format_str){};
 	void beginSession(MenuItem* navigatedBackwardFrom) override { Selection::beginSession(navigatedBackwardFrom); }
 
-	bool mayUseDx() { return !soundEditor.editingKit(); }
+	bool mayUseDx() { return !soundEditor.editingKit() && soundEditor.currentSourceIndex == 0; }
 
 	void readCurrentValue() override {
 		int32_t rawVal = (int32_t)soundEditor.currentSource->oscType;