Have some C++ file in the project

Rename gtk-gui.c to qt-gui.cpp, and make the necessary changes so that
the project still builds.

Signed-off-by: Alberto Mardegan <mardy@users.sourceforge.net>

display-gtk.h

@@ -8,6 +8,10 @@
 #include <gdk/gdkkeysyms-compat.h>
 #endif
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 extern GtkWidget *main_window;
 
 /* we want a progress bar as part of the device_data_t - let's abstract this out */
@@ -117,4 +121,8 @@ GError *uemis_download(const char *path, progressbar_t *progress, GtkDialog *dia
 /* from planner.c */
 extern void input_plan(void);
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif