"edit-paste-as-new-image-short" and "vectors-selection-to-vectors-short" were
just duplicate of the action named the same, except for the "-short" suffix, and
the only point was to have different labels.
Not though that this time, it was not enough to conclude that the action in a
menu shoud have the short variant. These were both used differently depending on
the menu.
Instead I added the concept of "label-variant" attribute in .ui menu files. When
the "long" variant is set, then we simply use the longer label.
There is still one more "-short" action: "tools-by-color-select-short", but I am
a still unsure how to handle this one.
