gui: add "New Sud" menu action, disable Start/Pause/Stop for an empty sud

File > New Sud loads an empty schedule (zero steps) onto the server,
same mechanism as Load but with a built-in empty document. Tracks
whether the loaded schedule actually has steps (sud_empty) separately
from whether a Sud is configured at all (sud_loaded) - Start/Pause/Stop
now require both, and the Automatic tab's forecast shows "No sud
loaded" instead of an empty plot while sud_empty.

client/brewpi.ui

@@ -3712,6 +3712,7 @@
     <property name="title">
      <string>&amp;File</string>
     </property>
+    <addaction name="actionSudNew"/>
     <addaction name="actionSudSave"/>
     <addaction name="actionSudLoad"/>
    </widget>
@@ -3765,6 +3766,14 @@
     <string>Stop</string>
    </property>
   </action>
+  <action name="actionSudNew">
+   <property name="text">
+    <string>&amp;New Sud</string>
+   </property>
+   <property name="toolTip">
+    <string>Replace the running Sud's schedule with an empty one</string>
+   </property>
+  </action>
   <action name="actionSudSave">
    <property name="text">
     <string>&amp;Save Sud...</string>