Migrate avatar CSS rules from editor to webodf.css

Each css file contained some of the rules required for the avatar display.
There are enough stylistic elements in webodf.css the avatars look strange
if the editor.css rules are not present.

Clean up this problem by merging all the rules into webodf core.
Improve avatar alignment to centre better with relation to the caret

The avatar was not staying well centred over the caret, and wouldn't move
vertically when the caret grew to match the text size. The easiest way to
ensure these stay properly aligned is to wrap both inside a sub-container.
Merge pull request #574 from peitschie/caret-avatar

Improve avatar alignment to centre better with relation to the caret
Add signalling for metadata changes to OdtDocument
and the Editor.
Add methods setMetadata and getMetadata to Editor.