validate exclusiveEdge and don't set it as top by default

[ikalco]

Describe your PR, what does it fix/add?

resolves #6987

Is there anything you want to mention? (unchecked code, possible bugs, found problems, breaking compatibility, etc.)

anchor is defined as a enum bitfield so 0 is valid

<enum name="anchor" bitfield="true">
 <entry name="top" value="1" summary="the top edge of the anchor rectangle"/>
 <entry name="bottom" value="2" summary="the bottom edge of the anchor rectangle"/>
 <entry name="left" value="4" summary="the left edge of the anchor rectangle"/>
 <entry name="right" value="8" summary="the right edge of the anchor rectangle"/>
</enum>

Is it ready for merging, or does it need work?

yes

[vaxerski]

thanks bae