c++ QgsLayoutItem metadata classes can directly register
a function which creates a QgsLayoutViewRubberBand for the item
subclass.
Python code cannot utilise this shortcut (due to inaccessibility
of forward declared gui classes from core Python classes), so
there's a separate gui registry utility class added for registering
prototypes for rubber bands for already registered item types.
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.
