Fix items in mode pie changing position by supporting more than 8 items in operator-enum pies

Now a 'More' item is added to the pie when there are too many items. It opens a sub-pie that contains the remaining items.
Note that this only touches operator-enum pies (like the object mode pie is), it is not a complete support for pies with more than 8 items. For this further design and code work would be needed, but this is too urgent to wait for that.

This is a better fix for T46973, should definitely be applied for 2.77 release.

Patch D1800 by myself with some edits by @campbellbarton, thanks!

source/blender/editors/interface/interface_intern.h

@@ -182,6 +182,9 @@ enum {
 
 #define PIE_CLICK_THRESHOLD_SQ 50.0f
 
+/* max amount of items a radial menu (pie menu) can contain */
+#define PIE_MAX_ITEMS 8
+
 typedef struct uiLinkLine {  /* only for draw/edit */
 	struct uiLinkLine *next, *prev;
 	struct uiBut *from, *to;
@@ -332,6 +335,10 @@ typedef struct ColorPickerData {
 } ColorPickerData;
 
 struct PieMenuData {
+	/* store title and icon to allow access when pie levels are created */
+	const char *title;
+	int icon;
+
 	float pie_dir[2];
 	float pie_center_init[2];
 	float pie_center_spawned[2];
@@ -603,6 +610,10 @@ uiPopupBlockHandle *ui_popup_menu_create(
         struct bContext *C, struct ARegion *butregion, uiBut *but,
         uiMenuCreateFunc create_func, void *arg);
 
+void ui_pie_menu_level_create(
+        uiBlock *block, struct wmOperatorType *ot, const char *propname, IDProperty *properties,
+        const EnumPropertyItem *items, int totitem, int context, int flag);
+
 void ui_popup_block_free(struct bContext *C, uiPopupBlockHandle *handle);
 
 int  ui_but_menu_step(uiBut *but, int step);