[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*