Add an icon to 'Open theme folder'

[isf63]

This was part of a bigger experiment to add "Open icons folder" and "Open cursors folder" context menu and double click, as #854 does for LXQt themes. The complexity seemed too much, but may be do-able in the future.

This PR trivially adds an icon.

[tsujan]

For reasons beyond this comment, the dialog of LXQt Appearances Configuration doesn't and can't follow symbolic icon colorization (although it enables/disables it correctly). So, it's preferable to avoid icons in it as far as possible.

In short, the reason is that it has to load all icon themes in its second page, and that ruins colorization inside the whole dialog.

[tsujan]

To make it clearer it with an image:

[isf63]

I guess I'm striking out with my recent PRs :-]

If you find time, please review the remaining lxqt/lxqt-themes#141. I will close the others fairly soon.