LibCore: Add Core::deferred_invoke_if(F, Condition)

This will invoke the function F only if the provided Condition is met,
otherwise the execution of the function F will be further deferred.

Userland/Libraries/LibCore/DeferredInvocationContext.h

@@ -13,8 +13,21 @@ namespace Core {
 
 class DeferredInvocationContext final : public Core::EventReceiver {
     C_OBJECT(DeferredInvocationContext)
+public:
+    bool should_invoke() const { return m_condition(); }
+
 private:
-    DeferredInvocationContext() = default;
+    DeferredInvocationContext()
+        : m_condition([] { return true; })
+    {
+    }
+
+    DeferredInvocationContext(Function<bool()> condition)
+        : m_condition(move(condition))
+    {
+    }
+
+    Function<bool()> m_condition;
 };
 
 }