Add preview to upgrade wizard

The tiddlers are wikified in the context of the host wiki, which means
that mutual transclusions between imported tiddlers won’t appear
correct in the preview.

core/ui/ImportListing.tid

@@ -7,6 +7,9 @@ message-$(payloadTiddler)$
 \define selectionField()
 selection-$(payloadTiddler)$
 \end
+\define previewPopupState()
+!!popup-$(payloadTiddler)$
+\end
 <table>
 <tbody>
 <tr>
@@ -26,14 +29,28 @@ selection-$(payloadTiddler)$
 <$checkbox field=<<selectionField>> checked="checked" unchecked="unchecked" default="checked"/>
 </td>
 <td>
-<$link to=<<payloadTiddler>>>
-<$text text=<<payloadTiddler>>/>
-</$link>
+<$reveal type="nomatch" state=<<previewPopupState>> text="yes">
+<$button class="btn-invisible btn-dropdown" set=<<previewPopupState>> setTo="yes">
+{{$:/core/images/right-arrow}}<$text text=<<payloadTiddler>>/>
+</$button>
+</$reveal>
+<$reveal type="match" state=<<previewPopupState>> text="yes">
+<$button class="btn-invisible btn-dropdown" set=<<previewPopupState>> setTo="no">
+{{$:/core/images/down-arrow}}<$text text=<<payloadTiddler>>/>
+</$button>
+</$reveal>
 </td>
 <td>
 <$view field=<<messageField>>/>
 </td>
 </tr>
+<tr>
+<td colspan="3">
+<$reveal type="match" text="yes" state=<<previewPopupState>>>
+<$transclude subtiddler=<<payloadTiddler>> mode="block"/>
+</$reveal>
+</td>
+</tr>
 </$list>
 </tbody>
 </table>