db manager: show error message in bar when adding a layer fails (fixes #8148)
Fix a crash after adding favourites to browser when the item is not yet populated

The model was getting broken due to list of children being overwritten and
it was sometimes crashing in the proxy model implementation.
[layout] Add layout measurement class QgsLayoutMeasurement

This class is used for storing measurements for a layout, and
supports storing a measurement unit alongside a measurement
length. A new class QgsLayoutMeasurementConverter handles
conversion between different units, including converting to and
from pixels via a specified DPI property.