Collection of patch post release of v0.2.3

[rocco8773]

This is an assorted collection of patches pose release of v0.2.3

• Reconnect the movementStarted and movementFinished signals between DriveControlWidget and DriveDesktopController so the motion group configuration widgets are properly disabled during the movement.
• Disable the control mode selection box during drive movement.
• Give the central motion group config widgets a fixed with so they do NOT stretch during a resize.
• Have the the plot in MGWidget retain its size when hidden (prevents stretching of other widgets).
• Have AxisControlWidget.target_position return None if the text box contains an empty string. This prevents a ValueError from being raised before the widget is fully initialized.
• Update GearValidButton to allow for more external control of setting icon and fixed sizes.
• Created QTAIconLabel class to streamline placing icons from qtawesome.
• If the transformer is changed from a valid one to and invalid transform, stop updating displays that require pinging the motor for position. If you do not do this, then a ValueError will be raised since there is no transform to convert between the drive coordinates and motion space coordinates.
• Add a simple legend to the MGWidget central builder layout.
• Add a legend to the display generated by MotionSpaceDisplay.