Fix accessibility of PSMTabBarControl

* it's now an AXTabGroup - thanks to AXTabs the position of the tab
  and total tab count are announced
* AXChildren shows only visible tabs, and all buttons (like overflow)
* AXValue + AXValueChanged notification enable announcing new tab name
  when switching between them with Cmd-Shift-[] (for unknown reason
  this does not work quite yet)
Add accessibility description to the overflow tab bar button
Fix accessibility of PSMTabBarCell

* implemented visual bounds - yields proper VoiceOver cursor bounds,
  proper order of presentation in sequentional navigation (VO-left and
  right arrows), and also enables touch exploration
* implemented AXValue - so that the selected tab can announce itself
  as such (AXFocused is for elements having keyboard focus, i.e. being
  first responder, which is something quite different than being
  selected from a list of other options)
Add accessibility description of the add tab button
Merge pull request #32 from dusek/accessibility

Fix accessibility