feat: add QFlowLayout, for variable width widgets (#271)

* feat: add QLayout

* add to docs

tests/test_flow_layout.py

@@ -0,0 +1,27 @@
+from typing import Any
+
+from qtpy.QtWidgets import QPushButton, QWidget
+
+from superqt import QFlowLayout
+
+
+def test_flow_layout(qtbot: Any) -> None:
+    wdg = QWidget()
+    qtbot.addWidget(wdg)
+
+    layout = QFlowLayout(wdg)
+    layout.addWidget(QPushButton("Short"))
+    layout.addWidget(QPushButton("Longer"))
+    layout.addWidget(QPushButton("Different text"))
+    layout.addWidget(QPushButton("More text"))
+    layout.addWidget(QPushButton("Even longer button text"))
+
+    wdg.setWindowTitle("Flow Layout")
+    wdg.show()
+
+    assert layout.expandingDirections()
+    assert layout.heightForWidth(200) > layout.heightForWidth(400)
+    assert layout.count() == 5
+    assert layout.itemAt(0).widget().text() == "Short"
+    layout.takeAt(0)
+    assert layout.count() == 4