Fix some Doxygen warnings

* gdk/gdkmm/general.h:
* gdk/src/pixbuf.hg:
* gtk/gtkmm/accelmap.h:
* gtk/src/application.hg:
* gtk/src/arrow.hg:
* gtk/src/button.hg:
* gtk/src/clipboard.hg:
* gtk/src/iconinfo.hg:
* gtk/src/menu.hg:
* gtk/src/menubutton.hg:
* gtk/src/plug.hg:
* gtk/src/recentaction.hg:
* gtk/src/scalebutton.hg:
* gtk/src/treeview.hg:
* gtk/src/uimanager.hg:
Correct parameter names in @param directives, remove extraneous @ and #,
change <xyz> to \<xyz\> when it shall not be interpreted by doxygen, etc.