Implement eth_sendTransaction for dApp-initiated transactions

Show a confirmation popup with tx details (from, to, value, data) and
password prompt when a dApp calls eth_sendTransaction. Sign and broadcast
the transaction in the background, returning the tx hash to the dApp.

src/popup/views/helpers.js

@@ -20,6 +20,7 @@ const VIEWS = [
     "settings",
     "transaction",
     "approve-site",
+    "approve-tx",
 ];
 
 function $(id) {