Bitcoin-Qt: massive options/settings handling rework

- add new options for database cache and script verification threads
- add new options for setting separate IPv6 and Tor proxy and show for
  which networks the base proxy is used currently
- add label which displays options that are overridden by command-line
  parameters
- proxy settings are not applied on-the-fly anymore and require a
  client restart (ApplyProxySettings() was removed and was not working
  very well anyway)
- re-work options reset and require a client shutdown (as it is much
  easier to do it this way without having to mess with what can be
  changed on-the-fly and what needs a restart anyway)
- options reset now writes default values for every single option
- when changing an option which requires a client restart display a 10
  second warning message in statusLabel (via a QTimer)
- when applying the changes via ok change that to a persistent message,
  which is displayed even after closing optionsdialog and re-open it,
  when no client restart was made
- remove dialog boxes used when changing language or proxy settings
- add setRestartRequired() and isRestartRequired() to OptionsModel and
  use the set function when updating options to signal OptionsDialog when
  a restart is needed
- resize optionsdialog a little and add some min sizes for certain GUI
  elements
- remove apply button from optionsdialog
- save and restore optionsdialog window position
- update nTransactionFee in QSettings with a set -paytxfee value when
  opening optionsdialog (I'm not sure about this yet, perhaps revert
  to not updating QSettings and just display current -paytxfee value in
  optionsdialog.)