EventHandlers content should have js coloring

[aeschli]

From @isidorn on December 6, 2016 11:16

Refs #16526

Event handler values should have javascript coloring. Currently it is colored as a string but all the js funcitonality is there. As a user would not expect this to work due to string coloring - thus it will be hard to find the nice intelli-sense support you have there

Copied from original issue: microsoft/vscode#16628

[aeschli]

Not easy to implement as this would drag in a whole JavaScript grammar. But not the original one, but one that uses HTML escaping rules.

[aeschli]

Would have to go into the textmate grammar we use (https://github.com/textmate/html.tmbundle)