Add simple timer functionality to Timeline

v2: Address style issues (by mosra), add documentation, and add
    currentFrameTime() for consistency with existing code.

src/Magnum/Timeline.cpp

@@ -61,4 +61,19 @@ Float Timeline::previousFrameTime() const {
     return duration_cast<microseconds>(_previousFrameTime-_startTime).count()/1e6f;
 }
 
+Float Timeline::currentFrameDuration() const {
+    if (!running) return 0;
+
+    auto now = high_resolution_clock::now();
+    auto duration = UnsignedInt(duration_cast<microseconds>(now-_previousFrameTime).count());
+    return duration/1e6f;
+}
+
+Float Timeline::currentFrameTime() const {
+    if (!running) return 0;
+
+    auto now = high_resolution_clock::now();
+    return duration_cast<microseconds>(now-_startTime).count()/1e6f;
+}
+
 }

src/Magnum/Timeline.h

@@ -134,6 +134,22 @@ class MAGNUM_EXPORT Timeline {
          */
         Float previousFrameDuration() const { return _previousFrameDuration; }
 
+        /**
+         * @brief Time since the last frame (in seconds)
+         *
+         * Returns time elapsed since nextFrame() was called. If the timeline is
+         * stopped, the function returns @cpp 0.0f @ce.
+         */
+        Float currentFrameDuration() const;
+
+        /**
+         * @brief Present time (in seconds)
+         *
+         * Returns time elapsed since start() was called. If the timeline is
+         * stopped, the function returns @cpp 0.0f @ce.
+         */
+        Float currentFrameTime() const;
+
     private:
         std::chrono::high_resolution_clock::time_point _startTime;
         std::chrono::high_resolution_clock::time_point _previousFrameTime;