Use AXValue of OakChooser's cells for accessibility announcements

Previously we used [NSCell stringValue], which is not suited for cells with
custom implementation for AXValue accessibility attribute.
Improve accessibility for OakBundleItemCell

More specifically item's displayed keyboard shortcut or tab trigger is now
announced by VoiceOver.

Implementation of accessibilityAttributeValue:forParameter: returns nil for
some attributes. This is a hack to avoid doing tedious adjustments of certain
parameterized attributes (like AXStringForRange, AXAttributedStringForRange,
AXBoundsForRange, AXRTFForRange) for string AXValue that is extended (appended)
by extra information at its end in accessibilityAttributeValue:. To get an idea
how tedious this is (even though our case is a bit simpler by not *prepending*
anything before the AXValue, and not being an editable field), look at
OakLinkedSearchFieldCell. This looses the ability for the user to interact with
the text, but considering how seldom this will be used (if at all), for now
this seems acceptable.

This is needed because VoiceOver asks for the *StringForRange attributes and if
implemented, will never query AXValue (which is where our custom behavior is).

In the future when tables will be migrated from cell-based to view-based ones,
the tab trigger or shortcut could be a separate NSView, which would probably
allow for much more elegant solution to this problem.
Implement basic accessibility for OakKeyEquivalentView

I experimented a lot with different combination of attributes, this one works
reasonably well compared to the others (even though it is less feature
complete). I may revisit experimenting with it at some later point.