[dbmanager] Don't set parent for dbmanager window, allows for minimising
window. Unminimise window when activating dbmanager.
[dbmanager] Don't trip up when loading a sql layer if sql ends with a ;
[dbmanager] Try to set sensible default column choice for sql window
Merge pull request #1582 from nyalldawson/db_manager_fixes

Usability improvements for db manager