citra-qt: Add customizable speed limit target
* Update SDL config for the new frame_limit option
* Made max lag time a function of target speed percent.
* Added a checkbox to enable/disable frame limiter
* UI: Prevent frame_limit from under/overflowing
* UI: Hide target speed percent when frame limiter is off
* Disable frame limit spin box when framelimit isn't enabled
Now based on std::chrono, and also works in terms of emulated time
instead of frames, so we can in the future frame-limit even when the
display is disabled, etc.
The frame limiter can also be enabled along with v-sync now, which
should be useful for those with displays running at more than 60 Hz.
