Here is my shortcut:
Activate:
"Display cover art" at
File > Options > Tag-Panel
and deactivate:
"Don't display first image from file directory as cover art" at
File > Options > Tags
Then you don't have to open the extended tag dialog at all, and you don't have to open the "add cover" file selection dialog when there is no file in the folder because you see that in the preview window.
Note the difference in the preview window:
If the cover image is already in the tag, image/jpeg (or other file extention) is displayed beside the preview.
If there is no tag-cover-image but a file in the folder, the filename is displayed beside the preview. In this case a right-click on the preview is enough to open the add cover file selection dialog.