fix: improved widget for mailbox selection

[madbob]

Diving into #8726 I stumbled upon MailboxInlinePicker, the only user of @riophae/vue-treeselect, a not-so-lightweight dependency which did not received any update in the last 7 years.

So I rewrote it using the more familiar NcSelect, which do not permit nested navigation and selection of subfolders. Let you evaluate if this is an acceptable loss.

This PR includes:

• major rewrite of MailboxInlinePicker, as specified above
• major rewrite of AccountDefaultsSettings, to adapt to the new MailboxInlinePicker, remove most of the duplicated code around, and separate the flags to disable the single inputs while saving (which was the original request of Default folder selection jumps #8726)
• minor review of ActionFileinto, also to be adapted to the new MailboxInlinePicker
• remove of all specific CSS style targeting vue-treeselect

My only concern is about the filter on mailboxHasRights, which has been here removed. It is not entirely clear to me what kind of check was here performed, but if it was required perhaps the same elaboration should be added also to Quick Folders selections (cfr. #13305) and similar folder/subfolder selections.

Fixes #8726

Note: if this PR is merged, I recommend to remove then the dependency from @riophae/vue-treeselect in package.json: I avoided to change that myself in order to not mess up with package-lock.json