Added keyboard shortcuts for flipping layer

[Neclusive]

Description of Changes

Added keyboard shortcuts for flipping current layer vertically and horizontally.
primary + F = flip horizontally
primary + Shift + F = flip vertically

resolves #2243 and #2235

Checklist

[x] This pull request follows the project's contribution guidelines

[cameronwhite]

Thanks for working on this!
The Image -> Flatten command also has the same Primary + Shift + F shortcut so we'd need to find some alternate shortcut for it (I think Primary + Shift + F makes sense for flip vertical since it's related to the horizontal flip)