a11y: Drop GtkAccessibleRange.get_minimum_increment()

a11y: Drop GtkAccessibleRange.get_minimum_increment()

MinimumIncrement is an AT-SPI-ism that has no counterpart in the ARIA
specification, so it should not live inside public API. Additionally,
it's not really a useful method because it collapses two values on the
adjustment API.

The only method in the GtkAccessibleRange interface should be the
set_current_value(), which allows ATs to control the current position in
a ranged widget.

The AT-SPI implementation can now use all the accessible properties,
including the VALUE_TEXT one, mapped to the Text property on the
AtSpi.Value interface.