Use Breeze style expanders for GTK2

[benedictleejh]

When I started using gnome-breeze, one thing I noticed, was that the expand and collapse icons for trees were a little out of sync with the original Breeze theme icons for those. So I decided to change that.

I've taken the original Breeze icons for expanding and collapsing trees, converted them to PNG, then filled in the colour to remove transparency in the icon shape, leaving only the background as transparent.

The icon size follows the Breeze icon size of 22x22, because using 12x12 fattens the line a little too much.

I've only done this for GTK2 because of 2 reasons: 1) I don't see an equivalent image in the GTK3 assets, 2) I don't have any GTK3 applications on my system that would allow me to check if the GTK3 version used the same expand/collapse icons as the GTK2 version.

Because this is a purely stylistic issue, I will understand if you choose to reject this pull request.