Improve key tooltip in KeySelectionDialog

Summary:
This adds a new field called "Status" into the tooltip which shows
whether the key is valid or not, and a reason why it's not valid. This
fixes the annoying problem when you select a key and it gets the "bad key"
icon without any explanation as to why it is considered bad.

Secondly the tooltip is now placed in a table, which makes it look
a little prettier.

Reviewers: aheinecke

Reviewed By: aheinecke

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D2012