Drag+Drop is broken

### Bug Summary

I can't drop anything anymore - dragging technically works, but when I try to drop whatever is being dragged, nothing happens.

#### Steps to reproduce

drag an instrument into the instrument panel

#### Reproducable on:

- Arch Linux with Qt5.15.2, Wayland (wlroots) and master (1.3.0-alpha.1.164+g877a93198)

#### Not reproducable on:

- Arch Linux with Qt5.15.2, XWayland (wlroots) and master (1.3.0-alpha.1.164+g877a93198)
- Ubuntu 21.04 with Qt5.15.2, Wayland (GNOME) and master
- Ubuntu 21.04 with Qt5.15.2, XWayland (GNOME) and master
- any with stable 1.2.2

#### Expected behavior

instrument gets dropped in the instrument panel

#### Actual behavior

after dropping, no instrument is added and the cursor starts behaving oddly (switches from dragging icon to pointer icon randomly)

#### Affected LMMS versions

Every master I tried to compile - not 1.2.2 stable though, even if compiled manually.

#### Logs

Click to expand

Notice: could not set realtime priority.

Lv2 plugin SUMMARY: 5 of 29  loaded in 7 msecs.

For details about not loaded plugins, please set

  environment variable "LMMS_LV2_DEBUG" to nonempty.

qt.qpa.wayland: Wayland does not support QWindow::requestActivate()

QDragManager::drag in possibly invalid state

Start by reproducing the drag-and-drop failure on Arch Linux with Qt 5.15.2 under Wayland, then compare it with XWayland and stable 1.2.2. Trace the instrument-panel drop behavior and use the reported QDragManager message as a lead; done means the instrument is added and the cursor remains normal after dropping.

Written by the indexing model from the issue text.