Add support for the Wayland security context protocol

This adds support for the Wayland security context protocol. It allows Waypipe to use a different Wayland socket with an attached security context. The compositor can use this to detect Waypipe connections and apply different rules. For example, it can restrict certain Wayland features or change server-side window decorations to reliably implement colored borders (#73). This feature is enabled by specifying the --secctx parameter with an arbitrary application ID.

Currently, the security context is already supported by KWin and Sway, and it can be easily enabled in other wlroots-based compositors.