This is implemented by allocating one of the (few!) remaining bits in
the cells’ attribute struct to indicate the cell should be “URL
highlighted”.
render_cell() looks at this bit and draws an underline using the color
from colors.urls (defaults to regular3 - i.e. yellow).
A new function, url_tag_cells(), iterates the currently detected URLs
and sets the new ‘url’ attribute flag on the affected cells.
Note: this is done in a separate function to keep urls_collect() free
from as many dependencies as possible.
urls_reset() is updated to *clear* the ‘url’ flag (and thus implicitly
also results in a grid refresh, _if_ there were any URLs).
We now exit URL mode on *any* client application input. This needs to
be so since we can’t know if the URLs we previously detected are still
valid.
This works just like show-urls-launch, except that instead of opening
the URL (typically using xdg-open), it is placed in the clipboard when
activated.
