Add categories / folders to structure template menus

[RealRaven2000]

For users with many templates, it may be beneficial to have folders / categories in the menu so the long list can be broken up into a number of smaller ones.

[RealRaven2000]

Some test versions - Screenshots:

To add a new popup, edit the new "Category" text item. Templates with the same category will be shown in the same popup. Everything (including popups) is added strictly in the order shown here, this currently includes any keyboard shortcuts.

Thunderbird 78-91: smartTemplate-fx-3.8pre24.zip
Thunderbird 68: smartTemplate-fx-2.17pre10.zip

To install, drag zip file into Add-ons Manager.

[RealRaven2000]

Added some improvements for the shortcuts and also changing categories / labels "on the fly" - you can now change a category and click on a different item, and the change will be saved automatically:

Thunderbird 78-91 (fully localized): smartTemplate-fx-3.8pre107.zip

To install, drag zip file into Add-ons Manager.

[RealRaven2000]

Implemented in version 3.8 - please note that for Thunderbird 68 you have to install new versions from the github releases page.

Unfortunately thunderbird.net does not accept updates for legacy versions anymore - they only support the new "Mail extension" format and our "mail extension with experimental APIs" which is a workaround for missing advanced features.