Graph widget: invalidate Cytoscape's canvas dimensions on resize

Telling Cytoscape explicitly about the resize is required, because it
doesn't monitor the dimensions of the viewport (which makes sense,
because it can be expensive). This fixes an issue which, since the last
Cytoscape update, caused the canvas to stay at the same size as before,
when the widget size was changed.
Graph widget: reduce the redraw timeout after resize to 100ms

This will cause a redraw to happen faster after a resize, but keeps
resizing still smooth. The former value of 400ms felt a bit to large
if one clicked right away into the graph after a resize. If Cytoscape
wasn't informed about the dimension change yet, the mouse positions were
wrong. This doesn't happen if the update is scheduled faster.