StatusQ/Utils: Reliable and generic way to bind to model's count

[micieslak]

Description

Models like ListModel and SortFilterProxyModel have handy count property. However that property is not part of QAbstractItemModel so shouldn't be accessed on models provided from outside in given component. Most proxy models doesn't have count method, in nim code it's present in most models but it's repeating code causing code duplication and risk of bugs.

In our codebase we also use rowCount directly but in most cases it's not reliable because expression won't be re-evaluated on count change.

That problem can be easily solved by generic attached type providing count for every QAbstractItemModel. Than it's enough to use myModel.ModelCount.count to have reliable, bindable count for every QAIM.