LibCore: Add ElapsedTimer::elapsed_milliseconds()

It does the same thing as elapsed(), but has a clearer name.
The hope is we'll move everything to the new name over time
and then remove the old name.

Userland/Libraries/LibCore/ElapsedTimer.cpp

@@ -29,7 +29,7 @@ void ElapsedTimer::reset()
     m_origin_time = {};
 }
 
-i64 ElapsedTimer::elapsed() const
+i64 ElapsedTimer::elapsed_milliseconds() const
 {
     return elapsed_time().to_milliseconds();
 }

Userland/Libraries/LibCore/ElapsedTimer.h

@@ -23,9 +23,15 @@ public:
     void start();
     void reset();
 
-    i64 elapsed() const; // milliseconds
+    i64 elapsed_milliseconds() const;
     Time elapsed_time() const;
 
+    // FIXME: Move callers to elapsed_milliseconds(), remove this.
+    i64 elapsed() const // milliseconds
+    {
+        return elapsed_milliseconds();
+    }
+
     Time const& origin_time() const { return m_origin_time; }
 
 private: