Adding hotkeys for authoring Touch UI. Fixes #248.

Added support for hotkeys in touch ui. 

ctrl+shift+e for edit layer
ctrl+shift+p for preview layer
ctrl+shift+a for annotate layer
ctrl+shift+d for developer layer
ctrl+shift+t for targeting layer
ctrl+shift+s to toggle sidepanel
Making this part of the OOTB `cq.authoring.editor` clientlib forces this feature to be "always on" when the ACS Commons package is installed, right? I wonder if we should make this have a category of `acs-commons.authoring.switchlayers`  which can in turn be optionally included via a "custom" clientlib like..

```
<jcr:root xmlns:cq="http://www.day.com/jcr/cq/1.0" xmlns:jcr="http://www.jcp.org/jcr/1.0"
    jcr:primaryType="cq:ClientLibraryFolder"
    categories="[cq.authoring.editor]"
    embed="[acs-commons.authoring.switchlayers"] />
```

This isnt too obtrusive of a feature so i could go either way; of course it could be problematic if a custom page implements its own overlapping touch keys.

WDYT?