Use thicker highlight border

[GrimPixel]

Is your feature request related to a problem? Please describe.
It is hard to notice which objects are selected as the border is thin.

Describe the solution you'd like
Have thicker borders.

[drwhut]

For v0.2.0, I was going to experiment with the selection highlight anyways, so I'll have a go at changing the width to see what works best, as well as trying out ideas like having a non-selected piece show a black highlight instead of no highlight.

I might also make these things settings in the options menu so players have the ability to choose what works for them.