Add support for setting multiple spellcheck languages

[RealRaven2000]

Thunderbird 102 supports using multiple spellcheck languages concurrently. This patch will support setting multiple languages such as:

%spellcheck(en,de)%

PS: You can still disable automatic spellcheck in a subsequent command %spellcheck(off)%
Related: [Bug 1761221]

[RealRaven2000]

Test version:

smartTemplate-fx-3.12pre57.zip

To install, download the archive file and drag into Thunderbird Add-ons Manager (don't unpack contents to install)

[RealRaven2000]

The previous version used legacy code, we can avoid this by using a compose API function:

https://webextension-api.thunderbird.net/en/latest/compose.html#getactivedictionaries-tabid

To Do: rewrite as API function called through notification API.

[RealRaven2000]

There was still a problem in Tb91 with the undefined global gActiveDictionaries object. Here is a fix that works also with Thunderbird 91.10:

smartTemplate-fx-3.12pre71.zip

To install, download the archive file and drag into Thunderbird Add-ons Manager (don't unpack contents to install)

[RealRaven2000]

Implemented in 3.12.1 on 28/06/2022