Don't use `constexpr` in Timeline.

There is no point in using this class at compile-time. Ever. Moreover
clang called through Emscripten complains that it cannot be constexpr.
Clang called normally doesn't complain, don't know why.

src/Timeline.h

@@ -86,12 +86,10 @@ class MAGNUM_EXPORT Timeline {
          * Creates stopped timeline.
          * @see start()
          */
-        inline constexpr explicit Timeline(): _minimalFrameTime(0), _previousFrameDuration(0), running(false) {}
+        inline explicit Timeline(): _minimalFrameTime(0), _previousFrameDuration(0), running(false) {}
 
         /** @brief Minimal frame time (in seconds) */
-        inline constexpr Float minimalFrameTime() const {
-            return _minimalFrameTime;
-        }
+        inline Float minimalFrameTime() const { return _minimalFrameTime; }
 
         /**
          * @brief Set minimal frame time
@@ -141,9 +139,7 @@ class MAGNUM_EXPORT Timeline {
          *
          * If the timeline is stopped, the function returns `0.0f`.
          */
-        inline constexpr Float previousFrameDuration() const {
-            return _previousFrameDuration;
-        }
+        inline Float previousFrameDuration() const { return _previousFrameDuration; }
 
     private:
         std::chrono::high_resolution_clock::time_point _startTime;