re-work optionsdialog to a tabbed UI based on an ui-file

- extend network options with a SOCKS version selection
- changing "Unit to show amounts in:" now also updates the unit used in
the transaction fee box
- string updates
- link Apply button and OK button when enabling or disabling them
- use LookupNumeric() from netbase to verify proxy address (via an
EventFilter)
- change proxy address field to QValidatedLineEdit and add visual
feedback
- add a status label used for displaying a message for invalid proxy
addresses
- allow usage of IPv6 address as proxy address
- added warning message when enabling / disabling SOCKS proxy