Add some padding to tray icons

[TimDeve]

As described in #259, the existing tray icons lack padding which means they appear larger than the average tray icon.

This PR adds some padding. Below are some example in context.

Before:

After:

[DeedleFake]

I think it was a bit too much. Now it looks slightly too small on GNOME compared to other icons, especially with the exit node disabled.

Edit: With a bounding box:

[TimDeve]

Right that's interesting, I think the size I picked falls in between two pixels on non high definition displays and it ends up looking smaller than expected.
Let me try to simulate that so I can find the right size.

[TimDeve]

I tried it with a standard definition VM and it does look like it lines up Gnome/Ubuntu:

So I guess the issue is more than 3rd party tray icons are sometime bigger so I changed it to something in between instead, does that look ok?

1080p:

4k (150%):