The "Proposed updates" checkbox must be changed

Binary package hint: update-manager

In Software Sources -> Internet Updates, there's a "Proposed updates" checkbox. Checking it on we include the "edgy-proposed" channel into our sources.list. I propose to change this checkbox (or even remove it completely).

Rationale:

- The proposed updates contains packages which are not fully tested, so an average user could unconsciously install software which leads to instability and system failure.
- The checkbox not warns the user about the above issue.
- The "edgy-proposed" is useful only when you want to test potentially harmful packages, in order to put it in "edgy-updates" or "edgy-security" when those packages are considered stable. An average user typically will not go into that business.
- The user which could be really interested in testing packages, typically has the knowledge of editing his sources.list accordingly.

I propose to do one of the following:

- To add the text "(not recommended)" to the "Proposed updates" checkbox label.
- To remove the "Proposed updates" checkbox completely.
- A pop-up window could appear when the user turn on the checkbox, containing a message like this: "This channel contains untested software which could be potentially dangerous to your system. Are you sure you want to turn it on?".
- To move the checkbox into another, less obvious location (for example, a new tab like the "Statistics" tab, with a warning message and a brief description about the "proposed" intends).