expose button to control displaying of remaining timer

include/devices.h

@@ -94,6 +94,15 @@ namespace Devices {
                 }
             }).build()
         )
+        .addSecondary(
+            Builder<Button>::instance(new Button{"Display remaining timer", "display_remaining_timer",
+                [](const char* msg) {
+                    if (strcmp("PRESS", msg) == 0 && !Display::Timer::tDisplayTimer.isEnabled()) {
+                        Display::Timer::tDisplayTimer.restart();
+                    }
+                }
+            }).build()
+        )
         .addSecondary(
             Builder<Button>::instance(new Button{"Update time", "update_time",
                 [](const char* msg) {

include/display.h

@@ -89,6 +89,7 @@ namespace Display {
                 }
             }, &ts, false,
             []{
+                if (!tDisplayTime.isEnabled()) return false;
                 tDisplayTime.disable();
                 return true;
             },