Wake all sleeping threads when leaving suspend

std::condition_variable::wait_for does not count time spent in sleep
mode, resulting in longer than expected waits.

include/util/prepare_for_sleep.h

@@ -0,0 +1,9 @@
+#pragma once
+
+#include "SafeSignal.hpp"
+
+namespace waybar::util {
+
+// Get a signal emited with value true when entering sleep, and false when exiting
+SafeSignal<bool>& prepare_for_sleep();
+}  // namespace waybar::util