[FIX] DendrogramWidget: Remove event filters before removing items

[ales-erjavec]

Issue

Fix what appears to be a quadratic time bottleneck in DendrogramWidget.clear

Alleviates gh-2663

Description of changes

• Remove event filters before removing items
  Each removeItem seems to be linear in number of installed filters in the whole scene -> so this would be quadratic for us (not accounting any other filters not part of DendrogramWidget).

Includes

• Code changes
• Tests
• Documentation

[ales-erjavec]

Benchmarked using:

import timeit

import numpy as np
from scipy.cluster.hierarchy import linkage

from PyQt5.QtWidgets import QApplication, QGraphicsScene, QGraphicsView
from Orange.widgets.unsupervised.owhierarchicalclustering import DendrogramWidget
from Orange.clustering import hierarchical as hier

app = QApplication([])
scene = QGraphicsScene()
view = QGraphicsView()
view.setScene(scene)

N = 15000

dw = DendrogramWidget()
scene.addItem(dw)

mat = np.random.exponential(0, size=N * (N - 1) // 2,)
Z = linkage(mat)
tree = hier.tree_from_linkage(Z)


def setup():
    dw.set_root(tree)


def run():
    dw.clear()


res = timeit.timeit(
    "run()", setup="setup()", globals=globals(), number=100
)
print(res)

Before: 17.68
After: 0.71

[codecov]

Codecov Report

Merging #4361 into master will decrease coverage by <.01%.
The diff coverage is 90.9%.

@@            Coverage Diff             @@
##           master    #4361      +/-   ##
==========================================
- Coverage   86.91%   86.91%   -0.01%     
==========================================
  Files         396      396              
  Lines       71991    71996       +5     
==========================================
+ Hits        62571    62575       +4     
- Misses       9420     9421       +1