Fixes block related memory leaks
Fixes block format buttons formats the wrong selection

To reproduce the error

* insert two paragraphs into an editable

"
Paragraph1
Paragraph2
"

* select both paragraphs and format them as h2
* click into the second paragraph and format as h3

The result before this fix would have been that in the last step both
paragraphs were formatted as h3.
Fixes block format buttons formats the wrong selection

To reproduce the error

* insert two paragraphs into an editable

"
Paragraph1
Paragraph2
"

* select both paragraphs and format them as h2
* click into the second paragraph and format as h3

The result before this fix would have been that in the last step both
paragraphs were formatted as h3.

Conflicts:
	src/plugins/common/format/lib/format-plugin.js
Adds changelog entry for previous fix
Updated artifact version for release
Updated artifact version
Fixes block format buttons format the wrong selection

Previous commit 62138f03f3132a0398ae13b68f26812a9779ac4e
didn't fix the problem.

Waiting until the selection has become valid fixes the problem.
Fixes block format buttons format the wrong selection

Previous commit 62138f03f3132a0398ae13b68f26812a9779ac4e
didn't fix the problem.

Waiting until the selection has become valid fixes the problem.

Conflicts:
	src/plugins/common/format/lib/format-plugin.js
Adds changelog entry for previous fix
Moves merged changelog entry to the correct file for this branch
Updates CSS to reflect most recent changes
Fixed simple demo.
Merge branch 'hotfix-0.21.x' of github.com:alohaeditor/Aloha-Editor into release-0.21.x
Merge branch 'hotfix-0.20.x' of github.com:alohaeditor/Aloha-Editor into release-0.20.x
Adds preprune and postprune for DOM nodes
Adds some documentation for new functions
Clarifies comment
Fixes undefined is not a function
Annotate UI Container API methods as @api methods and document them.
s/Button/ToggleButton/ to unbreak the 'align center' and 'remove alignment' buttons.