Allow showevent to work with objects

As per #382 

Allow showing and hiding of elements by object value:
```
<group showevent="status" showeventarg="level" showvalue="['1','2']">
  <status event="status">Status Test</status>
</group>
```
Or maybe:
```
<group showevent="status.level" showvalue="['1','2']">
  <status event="status">Status Test</status>
</group>
```