portal: implement window capture

wire/jay_compositor.txt

@@ -78,6 +78,11 @@ request take_screenshot2 {
     include_cursor: u32,
 }
 
+request select_toplevel {
+    id: id(jay_select_toplevel),
+    seat: id(wl_seat),
+}
+
 # events
 
 event client_id {

wire/jay_screencast.txt

@@ -43,6 +43,10 @@ request release_buffer {
     idx: u32,
 }
 
+request set_toplevel {
+    id: id(jay_toplevel),
+}
+
 # events
 
 event plane {

wire/jay_select_toplevel.txt

@@ -0,0 +1,3 @@
+event done {
+    id: id(jay_toplevel),
+}

wire/jay_toplevel.txt

@@ -0,0 +1,5 @@
+request destroy {
+}
+
+event destroyed {
+}