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.

In addition, all avatar rules have been migrated from editor.css into 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.