BUG: You can't stop dragging Tux in the level editor

[Alzter]

SuperTux Classic version: v0.4.2
System information: HTML

Describe the bug
When you drag Tux in the level editor, letting go of the mouse button does not release Tux. He is permanently stuck being dragged. You need to Save and Quit the editor to stop the bug. Toggling edit mode doesn't release Tux.

To Reproduce
Steps to reproduce the behavior:

1. Go to the Level Editor
2. Edit a level
3. Left click Tux to drag him
4. Try letting go of left click
5. You can't stop dragging Tux.

Expected behavior
Letting go of the left mouse button should release Tux from dragging in the level editor.