Installer: Refactor to use @import rather than concatting files

[Foldex]

Fixes #180
Partially Fixes #178
Partially Fixes #177

General

web has been renamed to adwaita

Installer

The adwaita folder is now copied to steamui/adwaita during install
Rather than concatting together a single file, the generated steamui/libraryroot.custom.css now contains multiple import statements pointing to adwaita through steamloopback.host ala:

@import url("https://steamloopback.host/adwaita/variants/base/_root.css");

Colorthemes

Colorthemes are now plain css and imported in libraryroot.custom.css

The old colortheme directory remains for compatability sake until I fix AdwSteamGtk to compensate.

Custom CSS

While we technically supported custom CSS before, it wasn't as discoverable. It is now an option in the installer.

Users can edit adwaita/custom/custom.css and provide the -cu flag to enable it.

Not the happiest with the placement of it residing inside the theme dir, any ideas?

Variants

There's really no need for base anymore.

While the underlying files will need a merge down the line, the -w flag is deprecated and now does nothing.

Remains for the sake of AdwSteamGtk compatability.

[Foldex]

Merged to latest master.

adwaita/custom/custom.css is now under custom/custom.css instead.

[Foldex]

Updated to address those issues.
I've also made the changes on AdwSteamGtk's side to support the css themes and new argument names, so we can just rip the bandaid on that when we merge this.