Improve the UI controlling the execution

[nthiery]

• Stop button next to the Run button when the computation is running
• Improve the visual feedback about the status (unavailable, waiting for kernel, ready, computing, done, ....); also put it to the to the right of the button rather than below to not waste vertical space
• Run all, Run all above, ...
• Keyboard shortcut to execute and move to the next cell
• Restart kernel (see #69)
• Could have an "About" button on the same line, playing the same role as the floating button on http://sage-package.readthedocs.io/en/latest/sage_package/sphinx-demo.html (after clicking Activate).

[mathieuboudreau]

Run all, Run all above, ...

+1, that feature would be very useful.

[deeplook]

I've noticed when reading the "Waiting for kernel..." message that it might be an idea to say what it is actually waiting for as it can make a huge difference if one is waiting for the kernel to load or to build. At least for users knowing the difference this might be helpful.