Add the start for a Wine->X11 DnD wrapper

src/wine-host/xdnd-proxy.h

@@ -0,0 +1,51 @@
+// yabridge: a Wine VST bridge
+// Copyright (C) 2020-2021 Robbert van der Helm
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+#include <memory>
+
+#include <windows.h>
+
+/**
+ * A simple wrapper that registers a WinEvents hook to listen for new windows
+ * being created, and handles XDND client messages to achieve the behaviour
+ * described in `WineXdndProxy::init_proxy()`.
+ */
+class WineXdndProxy {
+   protected:
+    WineXdndProxy();
+
+   public:
+    /**
+     * Initialize the Wine->X11 drag-and-drop proxy. Calling this will hook into
+     * Wine's OLE drag and drop system by listening for the creation of special
+     * proxy windows created by the Wine server. When a drag and drop operation
+     * is started, we will initiate the XDND protocol with the same file. This
+     * will allow us to drag files from Wine windows to X11 applications,
+     * something that's normally not possible. Calling this function more than
+     * once doesn't have any effect, but this will still be called from every
+     * plugin host instance. Because of that we'll use a global Win32 mutex to
+     * ensure that this is done from only one thread at a time.
+     */
+    static WineXdndProxy& init_proxy();
+
+   private:
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wignored-attributes"
+    std::unique_ptr<std::remove_pointer_t<HWINEVENTHOOK>,
+                    std::decay_t<decltype(&UnhookWinEvent)>>
+        hook_handle;
+#pragma GCC diagnostic pop
+};