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