Code review comment for lp:~charlesk/indicator-session/lp-1024395

I'm using GTK_ICON_SIZE_BUTTON for the IndicatorObjectEntry's icon (ie, the one that gets shown in the bar) and GTK_ICON_SIZE_MENU for the menuitems.

many of the IndicatorObjectEntry's icons use indicator-image-helper's hardcoded 22 pixels. I went with GTK_ICON_SIZE_BUTTON because its default pixel value of 20 was the closest to that number so that there would be some consistency between IOE icon sizes.