Merge #17587: gui: show watch-only balance in send screen

4a96e459d7 [gui] send: show watch-only balance in send screen (Sjors Provoost)
2689c8fd71 [test] qt: add send screen balance test (Sjors Provoost)

Pull request description:

  Now that we can create a PSBT from a watch-only wallet (#16944), we should also display the watch-only balance on the send screen.

  Before:
  <img width="1008" alt="before" src="https://user-images.githubusercontent.com/10217/69533384-030e9180-0f78-11ea-9748-c32c957e822e.png">

  After:
  <img width="1009" alt="Schermafbeelding 2019-11-26 om 11 44 17" src="https://user-images.githubusercontent.com/10217/69622879-19811f80-1042-11ea-8279-091012f39b38.png">

  I added a test to check the balance on the send screen, but it only covers regular wallets. A better would add a watch-only only wallet.

ACKs for top commit:
  meshcollider:
    utACK 4a96e459d7
  jb55:
    utACK 4a96e459d7
  promag:
    reACK 4a96e45, rebased and label change since last review.
  instagibbs:
    code review and light test ACK 4a96e459d7

Tree-SHA512: 4213549888bd309f72bdbba1453218f4a2b07e809100d786a3791897c75468f9092b06fe4b971942b1c228aa75ee7c04971f262ca9a478b42756e056eb534620

src/qt/forms/sendcoinsdialog.ui

@@ -1190,7 +1190,7 @@ Note:  Since the fee is calculated on a per-byte basis, a fee of "100 satoshis p
         <number>3</number>
        </property>
        <item>
-        <widget class="QLabel" name="label">
+        <widget class="QLabel" name="labelBalanceName">
          <property name="sizePolicy">
           <sizepolicy hsizetype="Preferred" vsizetype="Fixed">
            <horstretch>0</horstretch>