connectiondialog: avoid reset when devices are added/removed

[ahunt]

Followup to #17 .

STR:

1. Select a device.
2. Connect a new serial device.

Expected: device from 1 remains selected.

Actual: the combox box resets to the first item in the list.

This may require using beginRemoveRows()/beginAddRows() instead of resetting the model - but the docs are not entirely clear about whether this is relevant for model updates.
https://doc.qt.io/qt-6/qabstractitemmodel.html#beginRemoveRows