qt: keep a list of requested payments

Keep a list of requested payments in the Receive tab so that a user can
recall previously created requests after closing their windows.

Currently this list is not stored between bitcoin-qt sessions. This can
be implemented later, but it is not clear where it should be stored as
I don't think it belongs in the wallet (maybe in QSettings?)
Add main-specific node state
Add ThreadGetMyExternalIP to net thread group

Fixes #3372 -- crash at shutdown.
Merge pull request #3276 from sipa/nodestate

Add main-specific node state & move ban score
Merge pull request #3373

80ecf67 Add ThreadGetMyExternalIP to net thread group (Gavin Andresen)
[Qt] misc small Mac related changes/cleanups

- cleanup Info.plist.in and specify high DPI mode enable command as per
  http://blog.qt.digia.com/blog/2013/04/25/retina-display-support-for-mac-os-ios-and-x11/
- move setting of QApplication::setAttribute() to bitcoin.cpp and add
  attribute for enabling use of high DPI pixmaps for Qt >= 5.1
- add missing setWindowTitle() on Mac
- cleanup Mac / non-Mac setup in bitcoingui.cpp
Merge pull request #3297

84f8551 [Qt] misc small Mac related changes/cleanups (Philip Kaufmann)
rpcwallet: use EnsureWalletIsUnlocked() where possible

- replaces a pwalletMain->IsLocked() check
- in keypoolrefill init kpSize to 0 as we have the logic to determine max
  kpSize in pwalletMain->TopUpKeyPool() anyway
Merge pull request #3178

f914c7a rpcwallet: use EnsureWalletIsUnlocked() where possible (Philip Kaufmann)
Merge pull request #3207

666893b qt: keep a list of requested payments (Wladimir J. van der Laan)