merge doublemelon (#2067)

non-exhaustive (but exhausting) list of changes:

* base laid for multiple window support, but will likely require more work to work correctly
* encapsulation of frontend state for proper multi-instance support
* (JIT still needs a fix for the NDS::Current workaround but we can get there later)
* new, more flexible configuration system

src/DSi.h

@@ -130,7 +130,8 @@ public:
     void ARM7IOWrite32(u32 addr, u32 val) override;
 
 public:
-    DSi(DSiArgs&& args) noexcept;
+    DSi(DSiArgs&& args, void* userdata = nullptr) noexcept;
+    //DSi() noexcept;
     ~DSi() noexcept override;
     DSi(const DSi&) = delete;
     DSi& operator=(const DSi&) = delete;