WD-specific fixes made. Note that some of these won't show up until we
actually switch over to WD.
Edits to 4.5 Animatable properties

--HG--
extra : rebase_source : 53a8a5d48ec15ec98fd194a87132c3bbfb478eed
Replace [web-anim] with [web-animations]
Remove orphaned reference to merge composition operation
Edits to 4.6 Animation of property types
Fix links to 'associated with a timeline'
Fix bibliography references for CSS transitions and transforms
Tidy up box formatting
Fix procedure for evenly distributing keyframes

Steps 2 and 3 of the existing algorithm ensure that offsets are specified for
the (perhaps temporary) first and last keyframes only if there are no keyframes
present with offsets of 0 and 1 respectively. This means that if keyframes with
offsets of 0 and 1 are present, but the first or last keyframes lack specified
offsets, step 4 will fail.

This patch fixes this bug by always ensuring that the first and last keyframes
have specified offsets, so that step 4 can assign offsets to any interior
keyframes with unspecified offsets.

This patch also removes the use of temporary keyframes. These are not required,
as if the last first or last keyframes already have specified offsets, no
interpolation to calculate offsets is required.
Remove logic from keyframe list normalization for multiple values for a single property

With the current keyframe syntax, it's not possible to specify multiple values for a single property.
Remove text for handling 'non interpolable' values.
Fix default addition procedure
Clarify text about clock values
Minor edits to 4.7 Custom effects
Rework summary of timing event issues
Remove annotation describing other possible event types
Add another alternative for events
Fix indent of event issue
Make getCurrentPlayers (both instances) use the player sequence number for sorting
Minor edits to 5.3 TimedItem interface