Change gamma slider limit to 500%

[heftymouse]

Is your feature request related to a problem? Please describe.
Being a fork primarily for speedrunning it makes sense that there'd be an easy way to change your gamma to 500%, which is what most runners use.

Describe the solution you'd like
Change the limit for the gamma slider from 100% to 500%

Describe alternatives you've considered
Suffer with having to edit options.txt

Additional context
None