quickFilters assistant triggered multiple times when dropping mails to tab

[RealRaven2000]

This is a regression following the recent necessary changed in QF 5.15.1 to avoid monkey patching internal functions of Thunderbird.

In order to fix issue #328 to allow moving mails from different folders via quickMove, the quickMove routine currently breaks up the array of mails of into single messages. This can be improved by grouping the mails based on their source folders instead.

[RealRaven2000]

Here is a version that gathers the "split" emails grouped by source folder and only brings up one assistant window (per source folder). In most of standard use cases this will be mail dragged from a folder to another folder / QF tab or mails dragged from one folder to the quickMove button. However it is possible to "gather" mails from multiple source folders with quickMove, in which case multiple assistant windows will be displayed by necessity.

QuickFolders-mx-5.15.2pre10.zip

To install this version, download the zip file and drag it into Thunderbird Add-ons Manager without extracting contents.