This should improve compilation speed.
Things changed:
* Prefer forward-declarations in headers.
* Move header-includes out of headers if possible.
* Move some functions definitions out of headers.
* Put some member variables into unique_ptrs (see Client).
New formspec elements:
- `scroll_container[<X>,<Y>;<W>,<H>;<scrollbar name>;<orientation>;<scroll factor>]`
- `scroll_container_end[]`
Other elements can be embedded in this element. Scrollbar must be placed manually.