snap: Make native wayland client support configurable

Make possible for users to enable the native wayland client by using
  snap set mailspring wayland-native=true

By default mailspring will use xwayland, but the support can be toggled
by users with the said command.

snap/hooks/configure

@@ -0,0 +1,7 @@
+#!/usr/bin/env bash
+set -eu
+
+wayland_native="$(snapctl get wayland-native)"
+if [[ -z "$wayland_native" ]]; then
+    snapctl set wayland-native=true
+fi

snap/launcher

@@ -2,7 +2,8 @@
 
 if [ -n "$WAYLAND_DISPLAY" ] && \
    [ "${DISABLE_WAYLAND:-0}" = 0 ] && \
-   [ "${GDK_BACKEND:-wayland}" = "wayland" ]; then
+   [ "${GDK_BACKEND:-wayland}" = "wayland" ] && \
+   [ "$(snapctl get wayland-native)" != "false" ]; then
   args="--enable-features=WaylandWindowDecorations --ozone-platform=wayland --use-gl=desktop"
 fi