refactor(model): collapse the thread list to a single column

Five columns answered through Qt::DisplayRole; one column cannot, and a card
needs every field at once, so each gets its own role. Qt::DisplayRole keeps
answering the subject, which is what keyboard search and accessibility read.

Three things change shape rather than moving. DateRole hands over the QDateTime
itself, since the card decides how much of a date it has room for and a
pre-formatted string takes that decision away from the delegate. The subject
loses its "(3)" message-count suffix, which the reply count on line 2 now
states. And the two per-column tooltips become one card-wide tooltip, because
the marks no longer have columns of their own to hover.

The build is red at this commit: the view and the delegates still name the
deleted Column enumerators and are rewritten in the commits that follow.

0 files changed, 0 insertions, 0 deletions