control-center network panel changes

[Matthias Clasen <matthias clasen gmail com>]

I'd like to ask for blanket forgiveness for changes in the
control-center network panel.
There will be several string and UI changes as we work hard to bring
this panel to an acceptable functionality level for 3.0, and asking
for approval for each individually is going to be too cumbersome.

I have already committed the following changes:
- Add Option buttons
- Shorten some strings to avoid resizing (Secure HTTP proxy -> HTTPS proxy)
- Add mnemonics


Sorry for the inconvenience,

Matthias