[Qt] massive options/settings rework (no core changes)

- add new options for database cache and script verification threads
- 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.)
[Qt] display native dir separators in select dadatir dialog

- fixes display on Windows now \ instead of / before
Merge pull request #3504

6a1bf00 [Qt] display native dir separators in select dadatir dialog (Philip Kaufmann)
Merge pull request #3347

7e195e8 [Qt] massive options/settings rework (no core changes) (Philip Kaufmann)
Add missing MacPorts dependencies to osx build docs
Merge pull request #3506

f5e9d98 Add missing MacPorts dependencies to osx build docs (Michael Ford)
qt: Move initialization/shutdown to a thread

Move AppInit2 and Shutdown to a thread.

This allows a more responsive splash screen, prevents 'process does not respond'
messages from the window system and will allow for showing a user friendly
window while shutting down.
qt: Show window while shutting down

Makes it clear to the user that the application is still wrapping up
and the computer should not be turned off until it is finished.
qt: Remove global references in bitcoin.cpp

Remove the need for global references `guiref` and
`splashref` by making the BitcoinGUI and SplashScreen
classes register for the UI interface signals themselves.
qt: Stop shutdown detection timer during shutdown

Stop the shutdown timer from exiting the main loop
when shutdown is already in progress.

Fixes seeming hanging window after typing 'stop' in debug console.

Also hide the debug console during shutdown as it is useless without
a core to connect to.