Move desktop notifications to its own header

We'll need to use this from the chainloader.

src/wine-host/meson.build

@@ -61,6 +61,7 @@ host_common_sources = files(
   '../common/logging/common.cpp',
   '../common/logging/vst2.cpp',
   '../common/audio-shm.cpp',
+  '../common/notifications.cpp',
   '../common/plugins.cpp',
   '../common/process.cpp',
   '../common/utils.cpp',

src/wine-host/xdnd-proxy.cpp

@@ -19,6 +19,7 @@
 #include <iostream>
 #include <numeric>
 
+#include "../common/notifications.h"
 #include "editor.h"
 
 using namespace std::literals::chrono_literals;