driver andorcam2: add support for i2c

could be useful some times...
driver semcomedi: optimisation, don't run writer thread if everything is preloaded

when writting data, if everything is preloaded, there is no need for a thread.

So we skip thread creation in this case. It should speed up things slightly only in the cases
of very small resolutions with small dwell times. In the other cases it just reduces CPU load.
gui: Fixed bug that prevented image data from being shown
Merge branch 'master' of vpn.delmic.com:odemis
gui overlay: add (back) TextViewOverlay.set_label()

This allows to the change the text of the label. Needed for displaying FPS.
gui overlay: simplify label management

make sure the default colour is _something_, I've picked white.
gui overlay: some more simplification for labels