Fix navigation by keyboard in GridView

[jeremypw]

Fixes #584 Fixes #388

• Correct test for Gdk.SHIFT_MASK in event.state
  • Use properties focused_column, focused_row and current_page_num to simplify and clarify changing page and focused item
  • Consistently use cardinal numbers (starting at 1) for page, column, row variables and map key (unless specifically ending index)
  • Column and row variables consistently refer to the current grid only
  • Restore focus after page change or focus out/in to the previously item
  • Lose some unnecessary code
  • Fix some code style issues

Supercedes #445

[jeremypw]

Investigating incorporating @danirabbit PRs using child-focus

[jeremypw]

Looks like child-focus works well within a single grid but there is no obvious way of having the appropriate child focused when moving to another grid without keeping track of the focussed row and focussed column as we do now. In which case, the advantages of changing seem minimal.

@danirabbit Is it OK to go ahead with the current fix and may be use child-focus later if a cleaner implementation is found?

[danirabbit]

That's the roadblock I ran into as well. I haven't looked at this branch yet but I agree that there's not much of an advantage to using child focus if you have to keep track of grid position anyways when switching pages