Refactor the dialog engine's event handling to make it easier to add new events in the future

- Focus event split into focus and defocus
- Scroll event added but not yet properly implemented

Also:
- Remove the useless (and ignored) clickable attributes from the schema
- Pave the way for controls other than fields to be able to recieve keyboard focus

rsrc/schemas/dialog.xsd

@@ -137,7 +137,6 @@
       <xs:attributeGroup ref="frame"/>
       <xs:attributeGroup ref="font"/>
       <xs:attribute ref="def-key"/>
-      <xs:attribute name="clickable" default="false" type="bool"/>
       <xs:attribute name="fromlist" default="none" type="xs:string"/>
       <xs:attributeGroup ref="rect-size"/>
     </xs:complexType>
@@ -161,7 +160,6 @@
         </xs:simpleType>
       </xs:attribute>
       <xs:attribute ref="def-key"/>
-      <xs:attribute name="clickable" default="false" type="bool"/>
       <xs:attribute name="num" use="required" type="xs:integer"/>
       <xs:attributeGroup ref="rect-size"/>
     </xs:complexType>