Drive: improve GUI look & feel

Typically in the proxy settings windows:

• Use radio buttons instead of a combo box for proxy settings
• Distinguish value and label in combo boxes (to be able to store 'http' while displaying 'HTTP', same for 'manual'/'Manual', 'system'/'System', ...)
• Display server and port next to each other
• Refactor create_proxy_settings_box in proxy_settings.py: find a better way than iterating on the fields and handling particular cases with if field_id == 'config' for instance

Then later we would like to have a unified settings menu (like the Dropbox one) with separate tabs for configuring:

• The proxy
• The home folder
• Notifications
• The account (token revocation)
• ...