all: implement screen locking

wire/ext_session_lock_v1.txt

@@ -0,0 +1,25 @@
+# requests
+
+msg destroy = 0 {
+
+}
+
+msg get_lock_surface = 1 {
+    id: id(ext_session_lock_surface_v1),
+    surface: id(wl_surface),
+    output: id(wl_output),
+}
+
+msg unlock_and_destroy = 2 {
+
+}
+
+# events
+
+msg locked = 0 {
+
+}
+
+msg finished = 1 {
+
+}