Optimize and fix up some SVGs

[MewPurPur]

Addresses some icons I missed in my SVG optimization PRs. A few of them are small fix-ups, like tiny lines in the middle of arcs or unintentional elliptical arcs where they meant to be circlular. This saves about 5 kB.

Also I changed the opacity of some icons slightly. Many icons used alphas like 50 or 100. When doing 50/255 for example, it can get thrown off and become something like 0.19608 when opened in an editor. The alpha values 51, 102, 153, and 204 result in no precision inconsistencies (as they are 0.2, 0.4, 0.6, 0.8) so I used them where the change isn't noticeable. Even if it's noticeable, it's probably for the better, as some people have complained about low opacity icons. Where not applicable (e.g. an alpha value of 20, which isn't close to any of these), I changed to use at most 3 digits of precision.

^ I don't think this needs to be enforced strictly, it's just a nice thing to do once IMO, and also artists probably often copy the opacity of existing icons. (or maybe it's just me doing things like that? 😅)

[Calinou]

This saves about 5 MB.

I think you meant 5 KB? 😛

[MewPurPur]

Fixed!

[MewPurPur]

Of course this went on to become more of a dumping ground for more tweaks and improvements, so it's more like 8 kB now.

[Calinou]

Tested locally, it works as expected. Icons still look fine in the editor.

Binary sizes before/after (Linux x86_64, stripped debug editor build):

170,677,168  godot.linuxbsd.editor.x86_64.master
170,669,360  godot.linuxbsd.editor.x86_64.pr

[akien-mga]

Thanks!