Add dialog allowing you to alter the scenario's graphics sheets from within the scenario editor

- You can copy/paste images into the sheet, or import/export to/from png files

Also:
- Picture controls in the dialog engine have a new "scaled" flag; if set, the picture will be scaled into the provided bounds rather than overflowing. Currently, only full sheets honour the setting.

rsrc/schemas/dialog.xsd

@@ -194,6 +194,7 @@
       <xs:attribute name="name" type="xs:token"/>
       <xs:attribute name="type" use="required" type="picttype"/>
       <xs:attribute name="custom" default="false" type="bool"/>
+      <xs:attribute name="scaled" default="false" type="bool"/>
       <xs:attribute name="framed" default="true" type="bool"/>
       <xs:attribute name="size">
         <xs:simpleType>