auto create actions for adding new items of the newly registered type
This avoids hard-coding in available item types into layout UI classes,
and allows designers to handle plugin-supplied item types
These classes are used for storing measurements for a layout
(QgsLayoutMeasurement), sizes for a layout (QgsLayoutSize)
and points in a layout (QgsLayoutPoint)
They support storing a measurement unit alongside a measurement
length/size. A new class QgsLayoutMeasurementConverter handles
conversion between different units, including converting to and
from pixels via a specified DPI property.
