Bitcoin-Qt: only use qApp for Q(Core)Application::instance()
Merge pull request #2448 from Diapolo/Qt_qApp

Bitcoin-Qt: only use qApp for Q(Core)Application::instance()
Bitcoin-Qt: fix opening verify message tab via main menu
Bitcoin-Qt: fix GUI after initial multi-wallet patch

- adds 6 methods in BitcoinGUI to access some actions needed by the new
  WalletView class
- updates WalletView class to use these instead of trying to duplicate
  these
- cleanup walletview.{cpp/h} and remove all unneeded stuff
- this fixes problems with tabs toolbar (#2451) and export broken (#2436)
- more details in #2447
Merge pull request #2452 from Diapolo/Qt_fix_GUI

Bitcoin-Qt: fix GUI after initial multi-wallet patch
Merge pull request #2450 from Diapolo/Qt_fix_VMTab

Bitcoin-Qt: fix opening verify message tab via main menu
Fix signed/unsigned comparison warnings
Shutdown cleanup prep-work

Create a boost::thread_group object at the qt/bitcoind main-loop level
that will hold pointers to all the main-loop threads.

This will replace the vnThreadsRunning[] array.

For testing, ported the BitcoinMiner threads to use its
own boost::thread_group.
Rename util.h Sleep --> MilliSleep

Two reasons for this change:
1. Need to always use boost::thread's sleep, even on Windows, so the
sleeps can be interrupted (prior code used Windows' built-in Sleep).

2. I always forgot what units the old Sleep took.
LoopForever and ThreadTrace helpers
Port Thread* methods to boost::thread_group
Clean up shutdown process
Have Qt poll for shutdown requested, the QT way.
Rework to be compatible with boost 1.40