Remove Experiment and use messageDisplay.open() instead

[jobisoft]

This is a proposal to free this extension from using Experiments, turning it into a pure WebExtension and no longer asking for the full system access permission.

The original implementation was using Gloda. The WebExtension API stopped using Gloda for searches in Thunderbird 91, because it was unreliable and only searched folders which have been indexed (and sometimes even not those). The WebExtension query is therefore a bit slower than Gloda. Specifying a preferred folder should give equally fast results.

Note: The core team is actively working on a new message database, which will speed up WebExtension queries as soon as it has been implemented (this requires no changes to this add-on).

Changes:

• background script has been turned into a module, to keeping code in separate files/modules
• creation of menus.onClicked listener has been moved out of the loop (was created multiple times)
• link handling logic was moved into cb_show_message_from_thunderlink() and cb_show_message_from_cbthunderlink()
• replace Experiment by messageDisplay.open()