compilation times improvement

[ammen99]

This PR aims to improve compilation times of Wayfire's core and plugins.

A benchmark on my own laptop (which uses AMD Ryzen 5600U) with 6C12T:

• Before: 1m1s, slowest file: scale.cpp which takes ~9s.
• After: 24s, slowest file: tile-plugin.cpp which takes ~6s.

This is achieved through a combination of techniques:

• Headers are shuffled around so that we include less header files overall.
• We pre-declare a few very frequently used templates (option templates).
• We split some of the header-only helpers (workspace-wall, move-drag-helper) to a separate cpp file and link the resulting library with the plugins that need it.
• We use a custom PCH solution for plugins (manually generating it because meson is not smart enough yet, see Shared precompiled header mesonbuild/meson#4350)
• We switch away from nlohmann/json and into jsoncpp (an unfortunate breaking change).

TODOs:

• Benchmark the impact of jsoncpp vs nlohmann/json
• Look into whether the plugin PCH cannot be easily solved on the meson side, upstream seems to not be against solving it properly, but I have no knowledge of meson's internals.
• Clean up the [DONOTMERGE] commits.
• Fix build order (wobbly vs plugins/common)