Merge bitcoin-core/gui#416: Add RPC setting

bd5c826a96 gui: add RPC setting (Sjors Provoost)

Pull request description:

  RPC access is disabled by default for the GUI.

  With the proliferation of third party desktop applications that use the Bitcoin Core RPC (e.g. Specter Desktop, Sparrow and Wasabi), this PR makes them slight easier to configure. It's no longer required to find and edit `bitcoin.conf` to add `server=1` to it.

  <img width="447" alt="Schermafbeelding 2021-09-02 om 14 25 58" src="https://user-images.githubusercontent.com/10217/131844201-be3b49a8-ae88-47e6-8992-e95ee6b70f69.png">

ACKs for top commit:
  hebasto:
    ACK bd5c826a96, tested on Linux Mint 20.2 (Qt 5.12.8):
  shaavan:
    reACK bd5c826a96
  promag:
    Code review ACK bd5c826a96. Just minor fixes to the .ui form since last review.

Tree-SHA512: ab377e2358826096b499013bc3a864b7b63dff9859e96041e93ff0897d2319a35e8b3adcfb8df5f83274466c83d040d4ea18c546699421425c835e6f42562ae0

src/qt/forms/optionsdialog.ui

@@ -33,7 +33,7 @@
           <string>Automatically start %1 after logging in to the system.</string>
          </property>
          <property name="text">
-          <string>&amp;Start %1 on system login</string>
+          <string>Start %1 on system &amp;login</string>
          </property>
         </widget>
        </item>
@@ -179,13 +179,23 @@
            <property name="sizeHint" stdset="0">
             <size>
              <width>40</width>
-             <height>20</height>
+             <height>40</height>
             </size>
            </property>
           </spacer>
          </item>
         </layout>
        </item>
+       <item>
+        <widget class="QCheckBox" name="enableServer">
+         <property name="toolTip">
+          <string extracomment="Tooltip text for Options window setting that enables the RPC server.">This allows you or a third party tool to communicate with the node through command-line and JSON-RPC commands.</string>
+         </property>
+         <property name="text">
+          <string extracomment="An Options window setting to enable the RPC server.">Enable RPC &amp;server</string>
+         </property>
+        </widget>
+       </item>
        <item>
         <spacer name="verticalSpacer_Main">
          <property name="orientation">