Add a 2-dimensional positioner box widget

Description

A widget to control two positioners (e.g. x and y motors) at once. Has tweak and step buttons, and other similar display items to the standard positioner box.

Related Issues

closes #358 (closed)

Type of Change

• Refactors existing positioner_box to extract some core logic into a base class
• Adds a positioner_box_2d widget

Potential side effects

PositionerBox and PositionerControlLine (along with the newly added PositionerBox2D) are all now exported from the bec_widgets.widgets.control.device_control.positioner_box module

Screenshots / GIFs (if applicable)

Definition of Done

• Documentation is up-to-date.