Enhancements and bug fixes for Ginga

- Added a grid-type workspace
  - "Window" menu renamed to "Workspace"
    - "Add Workspace" item now creates a new sub-workspace of "main" which is
        user-selectable type (grid or tabs)
  - "Channel" menu updated
    - "Add Channel" now has option to select workspace for new channel
    - New "Add Channels" option to add multiple channels in one go
      - useful for populating a grid

- Fixed a bug where in Qt version of Header plugin the FITS header was not
   sorted according to the default order in the file
   - temporarily disabled sorting by keyword until I figure out this bug