Use mono-spaced font for line-break detection tests

The line detection tests rely on text wrapping at a particular point in
order to function. Enforcing a monospace font and using the em value for
paragraph width measurements ensures (relatively) consistent behaviour
across different systems, irrespective of the system fonts available.
Merge pull request #575 from peitschie/line-tests

Use mono-spaced font for line-break detection tests
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.
Make caret and avatar overlays rather than inline content

Having the caret and avatar display inside the cursor node causes text
wrapping and other element shifts to occur in some browsers (mainly FF)
due to there being layout impacts from the almost-but-not-quite 0-width
cursor element.

Moving to non-inline overlays for these bits of chrome allows the cursor
to be hidden completely.

TODO
This commit introduces a regression when composing text using an IME
as the overlay doesn't advance properly during composition.

Fixes #86 & #49
Fix caret placement when composing text via the IME

If the cursor node itself is visible, use the bounding rectangle from that
as the highest priority for the caret position.