ENH: Update X-Axis from QDateTimeEdits

[zdomke]

This PR gives functionality to the QDateTimeEdits in the Axes tab of the Archive Viewer:

The values in the QDateTimeEdits will be updated to reflect changes to the plot's X-Axis. Changing the values in these widgets also makes the expected change to the plot's X-Axis.

Caveats:

• If the user is editing a QDateTimeEdit's value, then it will not reflect changes to the X-Axis. This would be disruptive and prevent the user from setting the desired range value.
• If the left timestamp is larger than the right timestamp, then the smaller value is set as the X-Axis range "minimum", regardless of being set in the "maximum" QDateTimeEdit. And vice versa.