Split more things into libraries

[glebm]

This does not fully address the issue of most everything being a giant library but it's a start.

Note that the libraries are OBJECT libraries, so they may (and currently do) contain references to missing symbols.

Also fixes the implementation of transitive dependency support for OBJECT libraries.

[AJenbo]

Splitting headless from engine and endian_write are the only things I found a bit odd.

Besides unit tests headless would also make sens if we implement a game server, maybe that gives some better context.

[glebm]

• endian_write: I wanted to split off the functions that depend on SDL
• Headless: yeah mostly for unit tests. There is a quite a bit of code that only references headless rather than the whole engine

[AJenbo]

Headless: yeah mostly for unit tests. There is a quite a bit of code that only references headless rather than the whole engine

sadly a lot graphics related code is spread all over the place

[glebm]

Rebased and improved OBJECT library support so now dependencies are placed neatly next to the target definitions.

This PR no longer contains any source changes, only the build system changes.

[AJenbo]

A bit annoying having to define the object bounds in a meta language (cmake), but it's nice that we get to carve this up in to more manageable chunks.