Basically only whitespace and renaming variables and functions. Should
be functionally completely equivalent to previous commit.

- Add a manifest for webext version and remove install.rdf
- Remove files & functions that do setup (bootstrap.js, ...)
- Keep functions that do actual cleaning work (browser.js -> cleanlinks.js)

Remove recursive functions, as content scripts are injected in every frame.

Add messaging between background and content script to send
cleaned-click events and numbers of links cleaned.

In background.js, update the count (on the badget text), notify the
user, and (TODO) track the cleaned links for potential future
whitelisting

Loading options and updating+storing options on change still to be done.

Requires storage permission. Options are stored as they are displayed in
the options page, and loaded based on the object types in the default
values.

All the main code is wrapped in the loadOptions.then() to be sure that
we load options from storage before anything else.

A message is sent to the background script to trigger a new loadOptions()
whenever new values are saved, so that it stays up to date.

Also repeat the dirty hack to handle links added in a document with
ajax, which consistss of running a clean again after repdelay seconds
after a click.

A new contextMenu item is required for this, as we can't intercept the
previous one anymore in web extensions.

The setup for this feature is a little contrived:
- copying to clipboard can not be done from a background script.
- on links or selections, the target text (link to be cleaned) is
  available from the background script where the contextMenu event is
  handled.
- otherwise, only the script in the page knows where the right click
  that triggered the contextMenu is, thus what the text is.

Thus, organise the flow thusly:
1. In injected content script, on right click, send text link around
   click position to background and wait for (optional) response.
2. In background script, if our contextMenu item is clicked, get sent
   text link and clean it (or use a.href or selected text), and send
   cleaned link as message reply
3. In injected content script, if there is a response to the message,
   copy it to the clipboard.

NB.  Are we leaving hanging promises around? Or do they get GC'd?
NB2. There might be a race condition when accessing lastRightClick, i.e.
     the saved text link message (content script (1.) -> background (2.)).
     If clicked too fast, maybe the message didn't arrive yet. To fix
     that, we probably would need to wrap lastRightClick in another
     promise.

Also fix gotarget comment

There is a browser preference that controls this:
"When you open a link in a new tab, switch to it immediately"

However this preference is not available through the browserSettings API:
https://developer.mozilla.org/en-US/Add-ons/WebExtensions/API/browserSettings

Also move the cleaning function to cleanlinks.js.

Perform the cleaning in the onRequest function by returning the
{redirectUrl: ...} object if we have a cleaned link.

Integrate as comments all fixes from the original XUL code in case those
bugs show up again.

The AMO parser was throwing a syntax error on parsing the previous
version for some reason. Logic remains the same: identify a link in a
jacascript:... text and keep just that.