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
* WebService: Verify username and token; Log errors in PostJson
* Fixup: added docstrings to the functions
* Webservice: Added Icons to the verification, imrpved error detection in cpr, fixup nits
* fixup: fmt warning
