Clarify Web Component Interface section

[dvdeurse]

Rewrote the section about the Web Component Interface:

• reuse typescript definition for each function
• be more expliciet about the promises
• clarify the target step in the playAction() function
• various smaller rewrites that should improve the level of understanding when reading the spec

Other things to mention:

• I integrated the work proposed in Clarify how a graphic should handle action method calls #14 into this pull request. When this one gets merged, the latter should be rejected.
• I changed two things in the typescript definitions:
  • added undefined as a possible value in the Promise of dispose()
  • removed skipAnimation field from the customAction() function as this was never in the text