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