For some reason, labels of buttons in popups/menus were clipped left.
Removed that function (was the only place were it was used), clipping text
by its left is really bad for its understanding!
Instead, extended clip_middle code so that it optionnally preserves a right piece of
the given string (like the shortcut part of a menu label). Think that's what makes
more sense here (note that most menus adapt themselves to their longest item,
we do not have much fixed-width ones, anyway).
Also tweaked a bit clip_middle logic, so that left part always have at least ten chars
(in addition to already existing 'min width' checks).
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