GraphicsLayout: Always call layout.activate() after adding items

Items added to a `GraphicsLayout` only learn their size information
after the internal `QGraphicsGridLayout` recalculates the layout.
This is happening as a slot in the Qt event queue.
Not having updated geometry bounds directly after adding an item
leads to multiple issues when not executing the Qt event loop
in time (see below). This commit fixes that by always calling
`layout.activate()` after adding items, updating item sizes
directly.

This is a follow-up to PR #1167, where introducing a direct call to
`processEvents` was suspected to be able to cause side effects.

Notifying @j9ac9k and @campagnola, as they were involved in #1167.

Fixes #8
Fixes #1136

pyqtgraph/graphicsItems/GraphicsLayout.py

@@ -134,6 +134,9 @@ class GraphicsLayout(GraphicsWidget):
         item.geometryChanged.connect(self._updateItemBorder)
 
         self.layout.addItem(item, row, col, rowspan, colspan)
+        self.layout.activate() # Update layout, recalculating bounds.
+                               # Allows some PyQtGraph features to also work without Qt event loop.
+        
         self.nextColumn()
 
     def getItem(self, row, col):