Merge branch 'hotfix-0.21.x' of ssh://github.com/alohaeditor/Aloha-Editor into hotfix-0.21.x

Conflicts:
	build/hotfix-0.21.x-changelog.md
[maven-release-plugin] prepare release alohaeditor-0.20.23
[maven-release-plugin] prepare for next development iteration
Preparing maven release
Post release cleanup
Updated version within hotfix branch
Updated version within hotfix branch
Merge branch 'release-0.21.x' of github.com:alohaeditor/Aloha-Editor into dev

Conflicts:
	pom.xml
Removes obsolete CSS rules
Adds labels to the image plugin url, title, width, height input fields
Adds german translation for image-plugin title and source inputs
Merge branch 'hotfix-0.21.x' of ssh://github.com/alohaeditor/Aloha-Editor into hotfix-0.21.x
Fixes missing css file breaks building
Added method getIds() to Registry for getting the registered ids (in order of registering).
Fixed ContentHandlerManager to use the contentHandlers in correct order
Fixed GenericContentHandler to return unmodified content instead of undefined in case it finds the class aloha-block
Added Tests for the ContentHandlerManager
Merge branch 'hotfix-0.21.x' of github.com:alohaeditor/Aloha-Editor into hotfix-0.21.x

Conflicts:
	build/hotfix-0.21.x-changelog.md
Cleaned up some code for a demo
Microoptimizations

Significantly speeds up deleteing and pasting content.
Microoptimization

Only check firstChild and lastChild in nodeList once.
Microoptimization

Speeds up delete ops significantly.
Adds clarification to API documentation