a11y: Checkbox lacks AT-SPI "checkable" state

Checkboxes lack the "checkable" AT-SPI state (ATSPI_STATE_CHECKABLE) on the accessibility layer on Linux.

A checked checkbox has ATSPI_STATE_CHECKED, but not ATSPI_STATE_CHECKABLE, which is inconsistent. (How can something that's not checkable be checked?)

Sample steps to reproduce:

1) run the qtbase tabdialog example (examples/widgets/dialogs/tabdialog/tabdialog)

2) switch to the "Permissions" tab which has 3 checked checkboxes

3) start Accerciser

4) in the treeview representing the a11y hierarchy of the tabdialog example in Accerciser, select the "Readable" checkbox

5) query states via Accerciser's IPython console

Actual result:

[<enum ATSPI_STATE_CHECKED of type Atspi.StateType>,
 <enum ATSPI_STATE_ENABLED of type Atspi.StateType>,
 <enum ATSPI_STATE_FOCUSABLE of type Atspi.StateType>,
 <enum ATSPI_STATE_SENSITIVE of type Atspi.StateType>,
 <enum ATSPI_STATE_SHOWING of type Atspi.StateType>,
 <enum ATSPI_STATE_VISIBLE of type Atspi.StateType>] 

The checkbox reports the CHECKED state, but not the CHECKABLE state.

Expected result:
ATSPI_STATE_CHECKABLE should be reported as well.