[load] handle webextensions

[JanMarvin]

Currently we neither write nor copy the webextension folder around.

Needs folders (could be copied over):

xl/webextensions/
xl/webextensions/_rels/

Needs in _rels/.rels (currently unused):

<Relationship Id="rId2" Type="http://schemas.microsoft.com/office/2011/relationships/webextensiontaskpanes" Target="xl/webextensions/taskpanes.xml"/>

Needs in Content_Types:

<Override PartName="/xl/webextensions/taskpanes.xml" ContentType="application/vnd.ms-office.webextensiontaskpanes+xml"/>
<Override PartName="/xl/webextensions/webextension1.xml" ContentType="application/vnd.ms-office.webextension+xml"/>

Though, at the moment I have only a single file with webextensions. Do I?