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This doesn't need to be a QObject anymore after rebasing this on
KExtraColumnsProxyModel.

This should be connected to the returnPressed signal that takes no args, so
the QLineEdit one.

Apparently things broke because the signal is overriden in
TodoViewQuickAddLine, overriding siganls is always a hassle.

Found while using KOrganizer.

Let the user edit start dates by right-clicking in the start date
column.  If the new start date is later than a pre-existing due date,
the due date is adjusted.

Note:  to preserve TodoView's ABI, I put the new popup menu in TodoViewView.

Use `standaloneMonthName()` to generate the month name in the Month
View's header, as is done by KOrganizer's date navigator, because
`QString::toString()` produces inappropriate month names for Slavic
languages in this context.

BUG: 453292

Related: calendarsupport!43

It is deprecated

Use KPluginMetaData and KPluginFactory instead

The Todo view lets the user "move" a todo by changing its due date-time.
If the todo has a start date-time, it allowed dtDue to move to a time
earlier than dtStart, which is disallowed by RFC 5545.  This patch
adjusts dtStart if necessary to make the todo valid.

Depends on akonadi-calendar!18.

Add an error signal for the previous use instead. Using a widget message
box isn't necessarily the desired way to communicate errors for all
applications. This matters particularly for QML-based apps like Kalendar.

TodoModel predates the availability of that (and probably also
QIdentityProxyModel), using those saves a ton of boilerplate here.

The dependency on KF5::ItemModels isn't entirely new, a number of
dependencies of this already use that anyway.