Wrong title for Xournal++ documents in KDE panel/task manager

[pglpm]

Affects versions :

• OS: Ubuntu 20.04LTS
• Desktop environment: KDE Plasma 5.18.8 (Qt 5.12.8)
• Which version of libgtk do you use: 3.24.20
• Version of Xournal++: 1.1.1+dev and 1.1.1
• Installation method: apt (ppa) and also deb-file (but same problem with compiled version)

Describe the bug
In the KDE panel/task manager, the information thumbnails for Xournal++ documents show the wrong title. More specifically, they acquire the title of the icon on which the pointer was right before. As an example, see attached snapshot: the title of the Xournal++ window should be "MAT110..." (as clear from the open window); instead it appears as "Wrong title for Xournal++...", which is the title of the Firefox window instead.

Note that the correct window title is reported for example by system monitor or by xwininfo from a terminal.

To Reproduce
Steps to reproduce the behavior:

1. Make sure you have a KDE panel where the task manager items are not grouped, and one or more Xournal++ documents open
2. Move pointer to a panel icon of some other program window (for example, Firefox). Wait for info thumbnail to appear.
3. Move pointer to the panel icon of a Xournal++ window. Wait for info thumbnail to appear.
4. Check title in info thumbnail.

Expected behavior
The title in the info thumbnail should be that of the Xournal++ window. Instead, the title of some other info thumbnail is reported.

Screenshots of Problem

Additional context
Possibly related to #1442 ? The various desktop, svg, and other files seem to be correct, for example they begin with "com.github.xournalpp.xournalpp". No other spurious desktop files related to Xournal++ are present in the file system.