Clean up and extend QAccessible::State.

Clean up and extend QAccessible::State.

The state should contain useful and clear information.
Some of the old enum/bitfield members were not really clear.
Make them follow Qt terminology and shift the burden of interpreting
them to the bridge.

Apart from the previous commit changing from enum to bitfield, these
flags have changed names:

unavailable -> disabled
mixed -> checkStateMixed
protected -> passwordEdit (in the last commit)

floating is completely removed, even MSAA documentation states it is unsupported.

Some new states have been added.
Documentation added.

Change-Id: I152256e77a061f28ee5780f527524c80a2c7e333
Reviewed-by: Jan-Arve Sæther <jan-arve.saether@nokia.com>