plugin API: use const char* insead of char*

I hope it doesn't change the ABI

v2: fix GSnull/zzogl/zerogs
v3: duplicated code on windows...

plugins/GSnull/Windows/GSwin.cpp

@@ -34,7 +34,7 @@ LRESULT CALLBACK MsgProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
     return 0;
 }
 
-int GSOpenWindow(void *pDsp, char *Title)
+int GSOpenWindow(void *pDsp, const char *Title)
 {
 	WNDCLASSEX wc = { sizeof(WNDCLASSEX), CS_CLASSDC, MsgProc, 0L, 0L,
 					GetModuleHandle(NULL), NULL, NULL, NULL, NULL,