Modes and Tools can now be installed, removed and updated without restart; tmp folders are dealt with much better; GUI improvements for the Contribution Managers

This PR would resolve [#2579](https://github.com/processing/processing/issues/2579), fix [#2606](https://github.com/processing/processing/issues/2606) and close [#2507](https://github.com/processing/processing/issues/2507).

The main features introduced in these commits are:

* Modes and Tools can now be added, removed and updated without a restart.
* In the unlikely chance that this fails to happen without a restart (and this happens only in Windows, not on Linux, and, although I'm not too sure, I think not on OS X either), a restart button is displayed from which Processing restart automatically.
* The tmp folders delete much better, and in case they don't, they are removed the next time Processing is run.
* Changes in the UI of the Contribution Managers have been implemented.