qt5: Ensure correct link order

If optional libs don't appear in QT_LIBS, they need to be listed first for
proper static linking.
qt5: fix a build issue with osx and qtdbus

See comment in notificator.cpp for details.
qt5: split out qt config and add qt5 support

Qt5 detection is a big ugly mess, but at least we can fence it off.
qt5: Use the info gleaned from configure for plugin importing
qt5: Use QT_SELECT for debian-based distros.

At least Debian/Ubuntu use 'qtchooser' for switching between qt4/qt5 binaries.
It is a wrapper for all qt tools, and calls the named tool of the default
version unless overridden by the -qt= option or QT_SELECT environment variable.
QT_SELECT is set by configure once the qt version has been chosen.

Take for example, moc.

$ which moc
    /usr/bin/moc

$ ls -go /usr/bin/moc
    lrwxrwxrwx 1 9 Jul  3 21:33 /usr/bin/moc -> qtchooser

$ qtchooser -print-env
    QT_SELECT="default"
    QTTOOLDIR="/usr/lib/x86_64-linux-gnu/qt4/bin"
    QTLIBDIR="/usr/lib/x86_64-linux-gnu"

$ QT_SELECT=qt5 qtchooser -print-env
    QT_SELECT="qt5"
    QTTOOLDIR="/usr/lib/x86_64-linux-gnu/qt5/bin"
    QTLIBDIR="/usr/lib/x86_64-linux-gnu"

$ moc -v
    Qt Meta Object Compiler version 63 (Qt 4.8.4)

$ QT_SELECT=qt5 moc -v
    Qt Meta Object Compiler version 67 (Qt 5.0.1)

This should be harmless elsewhere.
qt5: tests: Make sure static plugin symbols are pulled in

Since we're now properly linking against static plugins for qt binaries,
we need to ensure that they're Imported properly.

Without these Imports, the linker drops some of the unused linked libs, causing
undefined symbols in QtCore.
qt5: Force macports default include path to be searched last

Allows outside includes to take precedence. In particular, this allows for
a specified Qt to be used without clashing with macports' headers.
qt5: fix --with-qt with no arguments
qt5: fix typo causing broken selection logic
Merge pull request #3346

42cc967 qt5: fix typo causing broken selection logic (Cory Fields)
371f3f6 qt5: fix --with-qt with no arguments (Cory Fields)
2691cbc qt5: Force macports default include path to be searched last (Cory Fields)
8b0f608 qt5: tests: Make sure static plugin symbols are pulled in (Cory Fields)
47ffb50 qt5: Use QT_SELECT for debian-based distros. (Cory Fields)
60dc589 qt5: Use the info gleaned from configure for plugin importing (Cory Fields)
c8ba8ef qt5: split out qt config and add qt5 support (Cory Fields)
c614bd7 qt5: fix a build issue with osx and qtdbus (Cory Fields)
328bdb1 qt5: Ensure correct link order (Cory Fields)
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.