Fixed window size doesn't play well with tiling window managers

[cjdenio]

Hello! I'd just like to mention that Opus's fixed window size makes it virtually unusable on a tiling window manager, where windows are automatically sized to fill the screen. It would be great if Opus's main window wasn't fixed sized, or even if there was an option to enable dynamic resizing. I'd be glad to open a PR for this if it's a desired feature. Thanks!