Use correct icon for preview toggle button

[weisJ]

The left and right icon were the wrong way round

[p0deje]

I actually thought it made more sense to flip them.

1. The left part of the icon shows a list with multiple entries. This resembles the main Maccy window.
2. The right part is large and shows the main contents of a single entry.

[weisJ]

I see your point. I guess it goes against my muscle memory because usually it is the other way round.