LibCore: Add factory to create and start a new ElapsedTimer

Simplifies one of the main usage patterns of the timer class.

Userland/Libraries/LibCore/ElapsedTimer.cpp

@@ -12,6 +12,13 @@
 
 namespace Core {
 
+ElapsedTimer ElapsedTimer::start_new()
+{
+    ElapsedTimer timer;
+    timer.start();
+    return timer;
+}
+
 void ElapsedTimer::start()
 {
     m_valid = true;

Userland/Libraries/LibCore/ElapsedTimer.h

@@ -13,6 +13,8 @@ namespace Core {
 
 class ElapsedTimer {
 public:
+    static ElapsedTimer start_new();
+
     ElapsedTimer(bool precise = false)
         : m_precise(precise)
     {