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

* feat: add QLayout

* add to docs

examples/flow_layout.py

@@ -0,0 +1,19 @@
+from qtpy.QtWidgets import QApplication, QPushButton, QWidget
+
+from superqt import QFlowLayout
+
+app = QApplication([])
+
+wdg = QWidget()
+
+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()
+
+app.exec()