ComboGrid: add trigger to delete from ComboGrid

ComboGrid: add trigger to delete from ComboGrid

when we have a combogrid that may be empty, we now show a
little 'x' where the user can delete the content

this is not shown when the field is not allowed to be empty

we add a new css for this because triggers need a background image
with a very specific layout:

110x22px which is 5 icons in one image for the various states
(normal, hover, active, focused, focused hover)

the icon is taken from the theme-crisp
form/tag-field-item-close.png but rearranged to fit the size

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>