Add new setting to control XKB modifier latching

Add new setting org.mate.accessibility-keyboard.stickykeys-latch-to-lock
to control whether latching is enabled together with sticky keys.

Author: cwendling

schemas/org.mate.accessibility-keyboard.gschema.xml.in

@@ -61,6 +61,10 @@
     <key name="stickykeys-enable" type="b">
       <default>false</default>
     </key>
+    <key name="stickykeys-latch-to-lock" type="b">
+      <default>true</default>
+      <description>Latch modifiers when pressed twice in a row until the same modifier is pressed again.</description>
+    </key>
     <key name="stickykeys-two-key-off" type="b">
       <default>false</default>
       <description>Disable if two keys are pressed at the same time.</description>