improved quickFilters integration with QuickFolders — button injection & toolbar fixes

[RealRaven2000]

There are a number of quirks affecting the "injected" command buttons of quickFilters that are displayed on the "current folder" toolbar of QuickFolders:

• the function toggleCurrentFolderButtons() which is triggered from QuickFolders on demand, now recreates any missing buttons on instances of the current folder toolbar (currently only 3pane).
• Resilient retry logic with adaptive timeout (10s → 30s).
• quickFilters_injectButton() helper added for consistent injection & legacy support.
• Buttons in 3-pane and single message tabs now respect quickFilters preferences.
• Single message tabs now show the quickFilters current folder icon correctly.

To do: resiliency testing with Thunderbird 143 beta!

[RealRaven2000]

Implemented in 6.8.3 - Published 22/09/2025