core: add missing mouse event "alt-ctrl-button2"

ChangeLog.adoc

@@ -29,6 +29,7 @@ New features::
 
 Bug fixes::
 
+  * core: add missing mouse event "alt-ctrl-button2"
   * core: remove trailing directory separators in home directories (issue #2070)
   * fset: allow filename starting with "~" in command `/fset -export`
   * irc: add missing tags on self action messages when capability echo-message is enabled (issue #2074)

src/gui/curses/gui-curses-mouse.c

@@ -74,6 +74,7 @@ char *gui_mouse_button_codes[][2] =
   { "*",  "alt-button2"        },
   { ")",  "alt-button3"        },
   { "8",  "alt-ctrl-button1"   },
+  { ":",  "alt-ctrl-button2"   },
   { NULL, NULL                 } };