Add an enable/disable switch for the temperature controller

New TempControllerBase.enabled (default False) forces y=0 in
process_pid() when off, same mechanism as the existing cooling
override - the controller tries not to drive the heater at all.

tasks/tempctrl.py: new 'Enable' recv command and 'Enabled' broadcast
on the TempCtrl channel. tasks/sud.py: SudTask now enables the
controller on any non-IDLE/DONE Sud state and disables it (forcing
power back to 0) on IDLE/DONE, so automatic mode owns it for the
duration of a run and hands back manual control once it stops/finishes.

gui: new "Enabled" checkbox on the Manual tab's Controller group,
synced from the server; the temp/heatrate setpoint controls are
disabled whenever the controller itself is disabled.

client/brewpi.ui

@@ -517,11 +517,27 @@
         <property name="title">
          <string>Controller</string>
         </property>
+        <widget class="QCheckBox" name="checkbox_tc_enable">
+         <property name="geometry">
+          <rect>
+           <x>0</x>
+           <y>25</y>
+           <width>141</width>
+           <height>25</height>
+          </rect>
+         </property>
+         <property name="toolTip">
+          <string>While disabled, the controller tries not to drive the heater (output forced to 0) and the setpoints below are inactive</string>
+         </property>
+         <property name="text">
+          <string>Enabled</string>
+         </property>
+        </widget>
         <widget class="QSlider" name="Slider_temp_soll">
          <property name="geometry">
           <rect>
            <x>20</x>
-           <y>70</y>
+           <y>100</y>
            <width>63</width>
            <height>281</height>
           </rect>
@@ -540,7 +556,7 @@
          <property name="geometry">
           <rect>
            <x>10</x>
-           <y>370</y>
+           <y>400</y>
            <width>67</width>
            <height>31</height>
           </rect>
@@ -556,7 +572,7 @@
          <property name="geometry">
           <rect>
            <x>80</x>
-           <y>370</y>
+           <y>400</y>
            <width>51</width>
            <height>31</height>
           </rect>
@@ -575,7 +591,7 @@
          <property name="geometry">
           <rect>
            <x>0</x>
-           <y>30</y>
+           <y>60</y>
            <width>141</width>
            <height>31</height>
           </rect>