Reset drag unfold timer when mouse exits Tree

[Akosmo]

Fixes: #115271
Kind of a regression fix of #110904 , since it was introduced during the dev phase of 4.6

Using _reset_drop_mode_over() on NOTIFICATION_MOUSE_EXIT, if enable_drag_unfolding and drop_mode_over (user is dragging TreeItem) are true. Originally, the timer for unfolding would start, but not stop if mouse had exited the tree. drop_mode_over is not nullptr, even after the mouse leaves Tree, and this variable is used for conditional instead drop_mode_flags, because the latter already resets to 0 through other means, meanwhile I only found drop_mode_over to reset inside _reset_drop_mode_over(), which originally is only called when drop_mode_flags = 0 during NOTIFICATION_DRAG_END.
drop_mode_over is reassigned once mouse enters Tree again, and _reset_drop_mode_over() already checks if the timer is running.

[Repiteo]

Thanks! Congratulations on your first merged contribution! 🎉