Fix possible freezes

Split the UI from the commands' execution:
- use callback functions fired when the child process exits instead of the old 'wait and see' approach and rearrange the building of the UI accordingly
- move a couple of functions from Utilities.js to extension.js
- get rid of the 'timeout' workaround (also from schema)

Fix methods' names: all but the exposed ones now are dashed
Fix a bunch of typos
Improve readability
Update manual

Remove 'Known issues' section and references to 'timeout'.
Update the description about the devices' availability reflecting the new behaviour of the extension.