Make preflight checklist manual buttons toggleable

Made it possible to set manual checklist buttons back to the pending
state, so that in case of an accidental click, the check can be undone
without resetting the entire checklist and having to go through it
again.

src/QmlControls/PreFlightCheckButton.qml

@@ -107,9 +107,9 @@ QGCButton {
             _telemetryState = _statePassed
             return
         }
-        if (manualText !== "" && _manualState !== _statePassed) {
+        if (manualText !== "") {
             // User is confirming a manual check
-            _manualState = _statePassed
+            _manualState = (_manualState === _statePassed) ? _statePending : _statePassed
         }
     }