Qt: new icons

- flat and b/w icons seems to be the new trend
- support for HiDPI/retina
- change icon sizes
- fix assets-attribution.md
- removed unused icons

src/Makefile.qt.include

@@ -212,6 +212,8 @@ BITCOIN_QT_H = \
 RES_ICONS = \
   qt/res/icons/add.png \
   qt/res/icons/address-book.png \
+  qt/res/icons/about.png \
+  qt/res/icons/about_qt.png \
   qt/res/icons/bitcoin.ico \
   qt/res/icons/bitcoin.png \
   qt/res/icons/bitcoin_testnet.ico \
@@ -222,11 +224,11 @@ RES_ICONS = \
   qt/res/icons/clock4.png \
   qt/res/icons/clock5.png \
   qt/res/icons/configure.png \
-  qt/res/icons/connect0_16.png \
-  qt/res/icons/connect1_16.png \
-  qt/res/icons/connect2_16.png \
-  qt/res/icons/connect3_16.png \
-  qt/res/icons/connect4_16.png \
+  qt/res/icons/connect0.png \
+  qt/res/icons/connect1.png \
+  qt/res/icons/connect2.png \
+  qt/res/icons/connect3.png \
+  qt/res/icons/connect4.png \
   qt/res/icons/debugwindow.png \
   qt/res/icons/edit.png \
   qt/res/icons/editcopy.png \
@@ -237,11 +239,12 @@ RES_ICONS = \
   qt/res/icons/eye_plus.png \
   qt/res/icons/filesave.png \
   qt/res/icons/history.png \
+  qt/res/icons/info.png \
   qt/res/icons/key.png \
   qt/res/icons/lock_closed.png \
   qt/res/icons/lock_open.png \
+  qt/res/icons/open.png \
   qt/res/icons/overview.png \
-  qt/res/icons/qrcode.png \
   qt/res/icons/quit.png \
   qt/res/icons/receive.png \
   qt/res/icons/remove.png \
@@ -256,7 +259,8 @@ RES_ICONS = \
   qt/res/icons/tx_mined.png \
   qt/res/icons/unit_btc.png \
   qt/res/icons/unit_mbtc.png \
-  qt/res/icons/unit_ubtc.png
+  qt/res/icons/unit_ubtc.png \
+  qt/res/icons/verify.png
 
 BITCOIN_QT_CPP = \
   qt/bitcoinaddressvalidator.cpp \