[frontend] Slow and Turbo modes (#3525)

Closes #3344

Adds slow and turbo modes with configurable speeds that can then be
toggled by the user. Behavior is:
- Standard/slow limit, toggle turbo = turbo
- Turbo limit, toggle turbo = standard
- Standard/turbo limit, toggle slow = slow
- Slow limit, toggle slow = standard

Enabling the turbo/slow mode enables the frame limiter unconditionally.

This has some conflicts with VSync. For example when I set my refresh
rate to 60hz and enable vsync, turbo mode does nothing. Not sure how to
go about fixing this, @MaranBr probably knows better the proper
solution.

Signed-off-by: crueter <crueter@eden-emu.dev>
Reviewed-on: https://git.eden-emu.dev/eden-emu/eden/pulls/3525
Reviewed-by: DraVee <dravee@eden-emu.dev>
Reviewed-by: CamilleLaVey <camillelavey99@gmail.com>

src/core/core_timing.cpp

@@ -201,7 +201,7 @@ u64 CoreTiming::GetClockTicks() const {
 
      if (Settings::values.sync_core_speed.GetValue()) {
          const auto ticks = double(fres);
-         const auto speed_limit = double(Settings::values.speed_limit.GetValue())*0.01;
+         const auto speed_limit = double(Settings::SpeedLimit())*0.01;
          return u64(ticks/speed_limit);
      } else {
          return fres;