Improve wlroots usage

[Gaspard--]

• instead of storing popups manually, use https://github.com/swaywm/wlroots/blob/master/include/wlr/types/wlr_xdg_shell.h#L175
• instead of using a "mapped" bool, and adding surfaces to the server right away, add surfaces when they are mapped. To know if a surface i mapped, one can use the "mapped" bools within the wlroots structs

Other things will be added as I notice them.