"Browse online content" formspec improvement

[Zughy]

Closes #10069

1. Descriptions have been moved in tooltips. It now says "Install mod name" and such when hovering on buttons
2. Irrlicht screws pixel art up when it's scaled with a decimal value, but apparently more details make it less evident
3. There were tons of table concatenations. I kept some, but in some other cases they made the code incomprehensible, so I got rid of them
4. Download icon is animated, but my PC is a toaster and I can't record a decent GIF
5. "View" string has been removed, it was too generic
6. Yes, main menu needs a complete overhaul, but in the meanwhile this allows me to drop a few decent icons and to make it less ugly
7. PLEASE don't lose yourself in tiny aesthetic details, this is an easy one. Ask yourself: is this better than before? Yes/no. If the answer is yes and the code is good, approve it. If no, junk it

To do

This PR is Ready for Review

How to test

Open MT, open CDB formspec, click icons

[ghost]

Cool icons and colors, I love them instead of the labeled buttons.

[rubenwardy]

These icons are a definite improvement

[rubenwardy]

There were tons of table concatenations. I kept some, but in some other cases they made the code incomprehensible, so I got rid of them

FYI: table concatenation is used because string concat (..) is expensive in Lua, especially with large strings.

Small uses of string concat are probably OK though. If you are concating lots of things, it may be better to use format - but a lot of the stuff here is fairly short I guess

[rubenwardy]

The internet icon doesn't look very clear to me. There's a lot of lines that all merge together. Could it be simplified a bit somehow?

[rubenwardy]

You've accidentally included the clear icon in this PR

[Zughy]

New icon, it should be clearer now

All requests have been addressed. I also used the new code style length in some cases, where 95 has been used as a soft limit (I've reached 100 characters, max)

[rubenwardy]

Yeah, that's better

[sfan5]

Looks good.