Add mode keys to move the cursor to the top, middle and bottom of the screen.

H/M/L in vi mode and M-R/M-r in emacs (bottom of screen not bound in emacs).

tmux.h

@@ -397,6 +397,7 @@ enum mode_key_cmd {
 
 	/* Copy keys. */
 	MODEKEYCOPY_BACKTOINDENTATION,
+	MODEKEYCOPY_BOTTOMLINE,
 	MODEKEYCOPY_CANCEL,
 	MODEKEYCOPY_CLEARSELECTION,
 	MODEKEYCOPY_COPYSELECTION,
@@ -406,6 +407,7 @@ enum mode_key_cmd {
 	MODEKEYCOPY_HALFPAGEDOWN,
 	MODEKEYCOPY_HALFPAGEUP,
 	MODEKEYCOPY_LEFT,
+	MODEKEYCOPY_MIDDLELINE,
 	MODEKEYCOPY_NEXTPAGE,
 	MODEKEYCOPY_NEXTWORD,
 	MODEKEYCOPY_PREVIOUSPAGE,
@@ -418,6 +420,7 @@ enum mode_key_cmd {
 	MODEKEYCOPY_SEARCHUP,
 	MODEKEYCOPY_STARTOFLINE,
 	MODEKEYCOPY_STARTSELECTION,
+	MODEKEYCOPY_TOPLINE,
 	MODEKEYCOPY_UP,
 };