Add icon and symlink for Seamly2d

[wilwarindi]

Add icon for the Flathub and appimage releases of Seamly2D

[morganist]

would it be possible to pixel-align the elements of the logo, or is it too complex?

[achadwick]

The top, bottom, and right hand side could align to pixel edges, I guess.

And it's possible that each stitch could be made to "catch" as much of one or more pixels as possible. Doing that makes things look less blurry inside a shape at tiny sizes. It can distort the shape of the icon though.

[morganist]

@wilwarindi you also forgot shadows under the logo in your Bambustudio pull request, consider adding them here. It adds that special layered look to the icon

[wilwarindi]

I'll try to simplify the shapes of the stitches to make it easier to align.

[morganist]

@wilwarindi well, i tried to align what i could myself, but it was pretty difficult... added shadows as well ;)