WAI-ARIA support for Tree Grid

[mar10]

https://www.w3.org/TR/wai-aria-practices-1.1/#treegrid

Note: this requires a different paradigm than the current ext-table/ext-gridnav implementation.

• Keyboard Interaction: Navigation Mode (Read-Only)
  Navigation Mode (Read-Only) is the default mode, and allows quick and intuitive navigation around the grid.
• Actionable Mode (Interactive) allows the interaction with other objects that might be found in the grid cells such as edit fields, links, etc.

f2, enter, esc are used to switch between modes.

See also handling of plain trees: #656

[mar10]

We consider the ext-table as a 'tree with tabbed titles', not a 'Tree Grid' in the WAI-ARIA sense.
So we currently do not distinguish between Navigation- and Actionable Mode.
But navigation should yet be intuitive and usable with screen readers:
http://wwwendt.de/tech/fancytree/demo/sample-aria-treegrid.html