Add start/stop all devices in workspace button in toolbar.

[srcejon]

This PR is to primarily add a "start/stop all devices in workspace" button, to the workspace toolbar.

I find this useful in a couple of cases:

• When recording/playing back data from multiple devices, it helps to sync them in time. (Not perfect, but better than by clicking start individually)
• For some modes, device settings don't need to be adjusted, so the device GUI can be hidden after setup - but start/stop button is still needed. Having it on the toolbar allows device GUI space to be used for something else.
• As with the latency example on the mailing list the other day, it can be useful to stop all devices at the same time to compare spectra.

In order to keep the state / colour of the toolbar button consistent with the state of the devices, I have added a signal in the device API (stateChanged) that is emitted whenever the device state changes.

This signal can also be used to eliminate the state polling that currently uses a timer in the device GUIs. Just as an example, I've updated the RTL SDR GUI to use this. If you think this patch is OK, then I can update some of the others to work the same.