[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.
[layout] Create a paper size registry for layout

Adds a new QgsPaperSize class and QgsPaperSizeRegistry singleton,
which stores and manages known page sizes
[layout] Start importing old classes