Add a "close plugin library" button (#2072)

* provides a "close plugin library" button

* starting from #1718 by @inmysocks
* possibly fixes all of #1718, #1597,  and #2067

* corrected code comment

* add back title

No idea why I overlooked this beforehand.

Next time I will not suggest such changes that have little to do with
the PR, but rather just comment the code.

* ah, sorry, "fixed" wrong spot

now

```
<$action-sendmessage $message="tm-load-plugin-from-library"
url={{!!url}} title={{$(assetInfo)$!!original-title}}/>
```

...should be back at the right spot.

* mhhh... still fixing the mess

load, unload, puh... let's see if I got it now

* added unloadIFrame and minor syntax fixes

core/ui/ControlPanel/Modals/AddPlugins.tid

@@ -90,6 +90,16 @@ $:/state/add-plugin-info/$(connectionTiddler)$/$(assetInfo)$
 </$list>
 \end
 
+\define close-library-button()
+<$reveal type='nomatch' state='$:/temp/ServerConnection/$(PluginLibraryURL)$' text=''>
+<$button class='tc-btn-big-green'>
+<$action-sendmessage $message="tm-unload-plugin-library" url={{!!url}}/>
+{{$:/core/images/chevron-left}} {{$:/language/ControlPanel/Plugins/ClosePluginLibrary}}
+<$action-deletetiddler $filter="[prefix[$:/temp/ServerConnection/$(PluginLibraryURL)$]][prefix[$:/temp/RemoteAssetInfo/$(PluginLibraryURL)$]]"/>
+</$button>
+</$reveal>
+\end
+
 \define plugin-library-listing()
 <$list filter="[all[tiddlers+shadows]tag[$:/tags/PluginLibrary]]">
 <div class="tc-plugin-library">
@@ -100,6 +110,10 @@ $:/state/add-plugin-info/$(connectionTiddler)$/$(assetInfo)$
 
 <$transclude/>
 
+<$set name=PluginLibraryURL value={{!!url}}>
+<<close-library-button>>
+</$set>
+
 <<display-server-connection>>
 </div>
 </$list>