Add choices to ComboBox and make phoebus support it

[coretl]

Extend https://github.com/epics-containers/pvi/blob/master/src/pvi/device.py#L114 to have choices like BitField has labels. If specified, the PV is assumed to be a string providing the current value, but the enum choices should be the ones in the ComboBox pvi structure rather than trying to get them from the PV metadata.

Add support for adding these to bob files. The combo widget supports this, you have to set:

    <items>
      <item>item 0</item>
      <item>Item 1</item>
    </items>
    <items_from_pv>false</items_from_pv>

[GDYendell]

e.g. PV with 120 enum entries, populate combo box entries with given choices instead of reading from mbbi fields, which are limited to 16 (?).

[GDYendell]

Added in #22