bad things (tm) can happen if we do a IMSG_GET hwen a page is loading. This fixes it by generating a new id for every request, so the current one (if there is one) can safely finish (stop_tab can take a while) while we're doing the next one.

Initially I took the IRI parser from gmid, stripped of the UTF-8 things and transformed back into a URI parser. Then I've added some functions to resolve URIs, but it wasn't enough. gmid parser is aimed at servers, and so it doesn't handle very well relative links. This makes use of the libphos uri parser, which was coded to follow the RFC more strictly, and to fully support all kinds of URIs. It'll need to be expanded in the future and be improved for sure

it bothers me to keep alive a pointer inside a struct tab.

to do that, move empty_vlist to wrap.c and provide a similar function for struct line*s.

- commands now take a struct window* instead of a raw tab - the struct ui_state is gone - using a window for the minibuffer too helps avoid code duplication - it will let us having more than one window on a tab! :DD It doesn't seem to have broken anything...