make workspace sidebar use a singleton instead of inferring

EpubContainer for gh-book and WorkspaceRoot for atc
add code to hide Book Picker title
if model does not have .findDescendantDFS then use the original model
for pointer events splice in the pointer model

this causes problems when filteredCollection tries to keep its
collection in sync
remove unused (and poorly defined) trigger

it should at the very least send @ as an argument
Merge branch 'master' into atc-root-singleton

Conflicts:
	scripts/gh-book/app.coffee
Merge branch 'master' into atc-root-singleton

Conflicts:
	scripts/gh-book/app.coffee
Merge remote-tracking branch 'origin/master' into atc-root-singleton
Merge remote-tracking branch 'origin/atc-root-singleton' into testing

Conflicts:
	scripts/gh-book/epub-container.coffee