Extend handling of app.open message in adblockplusui

[greiner]

Background

The message responder already contains an "app.open" message for opening pages such as the options page so it'd make sense to get rid of handling of "add-subscription" and extend "app.open" instead. Furthermore the current options page introduced the "focus-section" message as part of #3024 which could also be added the same way.

What to change

In messageResponder.js:

• Remove the handling of "add-subscription"
• Let "app.open" accept the additional message properties "focus" and "payload" where "payload" is optional data that should be forwarded to the specified page
• Currently, the following values are in use:
  • "add-subscription" with the payload containing a "url" and "title" property describing the subscription that should be added
  • "configure-notifications"

Notes for dependency update

This change removes the handling of the "add-subscription" and "focus-section" messages with "app.open". Therefore any occurence of those messages needs to be adapted accordingly.
Furthermore the optional callback parameter for ext.showOptions() can be removed since this was the only way it was used so far.

[greiner]

Closing this ticket in favor of ​webext#76.