Allow to filter issue table by issue type

[TheOneRing]

Add the ability to filter by issue type to the context menu of the issue type column header.

[TheOneRing]

#9010 ads the status to the table.
Filtering is still missing.

I suppose we only need filtering in the issue widget not the protocol widget.

The filter should be added to the header context menu.

The Model: https://github.com/owncloud/client/blob/2.9/src/gui/models/protocolitemmodel.cpp
The context menu:

• https://github.com/owncloud/client/blob/2.9/src/gui/issueswidget.cpp#L104
• https://github.com/owncloud/client/blob/2.9/src/gui/protocolwidget.cpp#L81
• https://github.com/owncloud/client/blob/2.9/src/gui/models/models.cpp#L80

[erikjv]

As far as I can see, OCC::Models::displayFilterDialog is used in two places. Do both need filtering by status?

The context menu is the same no matter which column you click, so I assume we need to show both the account filter, and the status filter. Also both filters should be usable independent of each other (e.g. "show all excluded for admin@")?

Or should there be only 1 active filter, depending on which column you click, that resets the filter for the other column?

The filtering can be done by a subclass of QSortFilterProxyModel, which asks for the UnderlyingDataRole of the item, and filters based on that.

[TheOneRing]

I think the filter only makes sense on the issue widget, the column is hidden on the protocol view.
It might come handy to allow to filter by account and status?
Whether we want to allow multiple accounts/states as filter, depends on the complexity of the solution.