urls: OSC-8 URLs can now optionally be underlined outside of url-mode

This patch adds a new configuration option,
‘osc8-underline=url-mode|always’.

When set to ‘url-mode’, OSC-8 URLs are only
highlighted (i.e. underlined) in url-mode, just like auto-detected
URLs.

When set to ‘always’, they are always underlined, regardless of mode,
and regardless of their other attributes.

This is implemented by tagging collected URLs with a boolean,
instructing urls_render() and urls_reset() whether they should update
the cells’ ‘url’ attribute or not.

The OSC-8 collecter sets this based on the value of ‘osc8-underline’.

Finally, when closing an OSC-8 URL, the cells are immediately tagged
with the ‘url’ attribute if ‘osc8-underline’ is set to ‘always’.

config.h

@@ -205,6 +205,11 @@ struct config {
     struct config_spawn_template notify;
     struct config_spawn_template url_launch;
 
+    enum {
+        OSC8_UNDERLINE_URL_MODE,
+        OSC8_UNDERLINE_ALWAYS,
+    } osc8_underline;
+
     struct {
         enum fcft_scaling_filter fcft_filter;
         bool allow_overflowing_double_width_glyphs;