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/qt/forms/sendcoinsentry.ui

@@ -67,6 +67,12 @@
          <iconset resource="../bitcoin.qrc">
           <normaloff>:/icons/address-book</normaloff>:/icons/address-book</iconset>
         </property>
+        <property name="iconSize">
+         <size>
+          <width>22</width>
+          <height>22</height>
+         </size>
+        </property>
         <property name="shortcut">
          <string>Alt+A</string>
         </property>
@@ -84,6 +90,12 @@
          <iconset resource="../bitcoin.qrc">
           <normaloff>:/icons/editpaste</normaloff>:/icons/editpaste</iconset>
         </property>
+        <property name="iconSize">
+         <size>
+          <width>22</width>
+          <height>22</height>
+         </size>
+        </property>
         <property name="shortcut">
          <string>Alt+P</string>
         </property>
@@ -101,6 +113,12 @@
          <iconset resource="../bitcoin.qrc">
           <normaloff>:/icons/remove</normaloff>:/icons/remove</iconset>
         </property>
+        <property name="iconSize">
+         <size>
+          <width>22</width>
+          <height>22</height>
+         </size>
+        </property>
        </widget>
       </item>
      </layout>