Factor out the framerate limiter into a class

Patch from @x-qq

src/pcedit/pc.main.cpp

@@ -21,6 +21,7 @@
 #include "cursors.hpp"
 #include "res_image.hpp"
 #include "prefs.hpp"
+#include "framerate_limiter.hpp"
 
 cUniverse univ;
 
@@ -163,8 +164,7 @@ void init_main_window (sf::RenderWindow& mainPtr, sf::View& mainView) {
 
 void handle_events() {
 	sf::Event currentEvent;
-	sf::Clock framerate_clock;
-	const sf::Int64 desired_microseconds_per_frame { 1000000 / 60 }; // us / FPS
+	cFramerateLimiter fps_limiter;
 
 	while(!All_Done) {
 		while(mainPtr.pollEvent(currentEvent)) handle_one_event(currentEvent);
@@ -172,9 +172,7 @@ void handle_events() {
 		redraw_everything();
 
 		// Prevent the loop from executing too fast.
-		const sf::Int64 remaining_time_budget = desired_microseconds_per_frame - framerate_clock.getElapsedTime().asMicroseconds();
-		if(remaining_time_budget > 0) sf::sleep(sf::microseconds(remaining_time_budget));
-		framerate_clock.restart();
+		fps_limiter.frame_finished();
 	}
 }