[pgAdmin][RM4938] Code refactoring of 'Columns' node

Hi Hackers,

Attached is the patch contains following fixes:

- RM #4938 Code refactoring of 'Columns' node.
- RM #4761 Wrong type when changing timestamp with time zone to
timestamp without a time zone.
- RM #4964 Column length is not being removed through the properties of
any table.
- RM #4965 Interval data type is not displayed in the properties section
of a table>columns.
- Change the label from 'Length and Precision' to 'Length/Precision and
Scale' for columns.
- The maximum length for datatype like interval, timestamp with time
zone, time with time zone, etc.. is 6. Set the max length to 6 instead of
10.

Please review and test it from the table and column dialogs.

--
*Thanks & Regards*
*Akshay Joshi*

*Sr. Software Architect*
*EnterpriseDB Software India Private Limited*
*Mobile: +91 976-788-8246*