5 button mouse support from b333rt in IRC with some edits for X11.

Tested in X11 where its fairly confusing.
buttons 4 and 5 are used for the wheel which is well known, but it seems 6 and 7 are used for horizontal scrolling, my mouse assigns the extra 2 buttons to events 8 & 9.

So the X11 events used for buttons called 4&5 in blender are 8&9 in X11.

The mouse buttons can be re-ordered like this once xorg starts (swaps 6,7 with 8,9)
  xmodmap -e "pointer = 1 2 3 4 5 8 9 6 7"

Couldn't test Win32, Apple not supported.
If someone wants to add horizontal scrolling its quite easy.

4 files changed, 13 insertions, 0 deletions

diff --git a/source/blender/editors/space_outliner/outliner.c b/source/blender/editors/space_outliner/outliner.c
index a3b47d505fd..a1fdbab9ccc 100644
--- a/source/blender/editors/space_outliner/outliner.c
+++ b/source/blender/editors/space_outliner/outliner.c
@@ -5049,6 +5049,8 @@ static char *keymap_mouse_menu(void)
 	str += sprintf(str, formatstr, "Left Mouse", LEFTMOUSE);
 	str += sprintf(str, formatstr, "Middle Mouse", MIDDLEMOUSE);
 	str += sprintf(str, formatstr, "Right Mouse", RIGHTMOUSE);
+	str += sprintf(str, formatstr, "Button4 Mouse ", BUTTON4MOUSE);
+	str += sprintf(str, formatstr, "Button5 Mouse ", BUTTON5MOUSE);
 	str += sprintf(str, formatstr, "Action Mouse", ACTIONMOUSE);
 	str += sprintf(str, formatstr, "Select Mouse", SELECTMOUSE);
 	str += sprintf(str, formatstr, "Mouse Move", MOUSEMOVE);
@@ -5071,6 +5073,8 @@ static char *keymap_tweak_menu(void)
 	str += sprintf(str, formatstr, "Left Mouse", EVT_TWEAK_L);
 	str += sprintf(str, formatstr, "Middle Mouse", EVT_TWEAK_M);
 	str += sprintf(str, formatstr, "Right Mouse", EVT_TWEAK_R);
+	str += sprintf(str, formatstr, "Button4 Mouse ", BUTTON4MOUSE);
+	str += sprintf(str, formatstr, "Button5 Mouse ", BUTTON5MOUSE);
 	str += sprintf(str, formatstr, "Action Mouse", EVT_TWEAK_A);
 	str += sprintf(str, formatstr, "Select Mouse", EVT_TWEAK_S);
 	
diff --git a/source/blender/makesrna/intern/rna_wm.c b/source/blender/makesrna/intern/rna_wm.c
index fff51ad8ade..0dd9e3aed42 100644
--- a/source/blender/makesrna/intern/rna_wm.c
+++ b/source/blender/makesrna/intern/rna_wm.c
@@ -46,6 +46,8 @@ EnumPropertyItem event_type_items[] = {
 	{LEFTMOUSE, "LEFTMOUSE", 0, "Left Mouse", ""},
 	{MIDDLEMOUSE, "MIDDLEMOUSE", 0, "Middle Mouse", ""},
 	{RIGHTMOUSE, "RIGHTMOUSE", 0, "Right Mouse", ""},
+	{BUTTON4MOUSE, "BUTTON4MOUSE", 0, "Button4 Mouse", ""},
+	{BUTTON5MOUSE, "BUTTON5MOUSE", 0, "Button5 Mouse", ""},
 	{ACTIONMOUSE, "ACTIONMOUSE", 0, "Action Mouse", ""},
 	{SELECTMOUSE, "SELECTMOUSE", 0, "Select Mouse", ""},
 
diff --git a/source/blender/windowmanager/intern/wm_event_system.c b/source/blender/windowmanager/intern/wm_event_system.c
index 40026d27bac..ebb7adc3cd5 100644
--- a/source/blender/windowmanager/intern/wm_event_system.c
+++ b/source/blender/windowmanager/intern/wm_event_system.c
@@ -1579,6 +1579,10 @@ void wm_event_add_ghostevent(wmWindow *win, int type, void *customdata)
 				event.type= LEFTMOUSE;
 			else if (bd->button == GHOST_kButtonMaskRight)
 				event.type= RIGHTMOUSE;
+			else if (bd->button == GHOST_kButtonMaskButton4)
+				event.type= BUTTON4MOUSE;
+			else if (bd->button == GHOST_kButtonMaskButton5)
+				event.type= BUTTON5MOUSE;
 			else
 				event.type= MIDDLEMOUSE;
 			
diff --git a/source/blender/windowmanager/wm_event_types.h b/source/blender/windowmanager/wm_event_types.h
index b331e036b9e..898c6358f3a 100644
--- a/source/blender/windowmanager/wm_event_types.h
+++ b/source/blender/windowmanager/wm_event_types.h
@@ -56,6 +56,9 @@
 		/* only use if you want user option switch possible */
 #define ACTIONMOUSE		0x005
 #define SELECTMOUSE		0x006
+/* Extra mouse buttons */
+#define BUTTON4MOUSE	0x007  
+#define BUTTON5MOUSE	0x008
 		/* defaults from ghost */
 #define WHEELUPMOUSE	0x00a	
 #define WHEELDOWNMOUSE	0x00b