add a 'lock channels together' function in left/right eq

[sadko4u]

Original feature request: https://github.com/lsp-plugins/lsp-plugins-para-equalizer/issues/6

this would be helpful for app's like EasyEffects that now has the functionality to show the native LSP window for EQ. my issue is that it uses the left/right window thus making it a bit annoying to use when im wanting to EQ both channels together (like in the 'stereo' plugin). do have it defaulted in left/right mode for the left/right for obvious reasons and do leave the stereo plugin for, again, obvious reasons.