give add/remove errors a GUI

previously we'd ignore the returned errors leaving the user hanging when adding a share fails and not even telling them why.

we now route errors on add/remove through somewhat generic kmessagebox-based reporting tech. unfortunately this is fairly excessive WRT string mapping because ksambashare sports one enum for everything so API-wise a call to save() could return UserShareAclOk but in practice that of course cannot happen because that'd mean nothing. as such many of the strings are overly generic because they cannot practically appear on weak context actions like save(). very meh.

this depends on the new lastSystemErrorString() API in KIO to stringify the majority of failure scenarios here, which come out of samba's 'net' command

trying to share /tmp now produces a kmessagebox informing the user that the dir may not be shared

BUG: 334618 FIXED-IN: 20.12

depends on frameworks/kio!97 (merged)