It already shows you every class and/or ID that is attached to the selected component. How much simpler can it be than that? Anything you select in the Overview pane or in the Visual Editor window responds by showing all CSS that affect that component. You don't need a button to click to open the CSS window, you have the entire bottom bar of the app to do that with which to me is a pretty large window. Although it's a bit hard to tell, the bottom bar that the HTML and Styles tabs are on is clickable. The tabs will also open the window, but they don't close it so that you can rearrange them if needed.
The CSS file names for every class are in gray to the right for every class/id that shows there. Need to get to that file? Click that grey name and poof that file opens in the Styles window as well. I see no better way for this to happen and can't see adding more buttons for something like this.
If I'm missing the point, please let me know, no offense meant, just not seeing the need for this as it already exists from what I'm seeing.
Need your new class or ID to go to a specific file? Then set that up in the Appearance window. Click any component and you'll see the settings on the right (as long as it isn't custom code that is). Now click the "Style Attribute" dropdown and tell it what you want it to do. It's pretty straight forward and allows you to add to already existing as well as new CSS files.
Everything you're asking for is already there as far as i can tell. Clarify it if I'm missing the point though.
