xdg-shell: Rename set_transient_for to set_parent

It's a confusing name that comes from the ICCCM. The ICCCM is best
forgotten about.

With the addition of the potential new "transient" role meaning a
parent-relative toplevel like a long-lived popup, used for e.g.
tooltips, the set_transient_for name will become even more confusing.
diff --git a/clients/window.c b/clients/window.c
index 4592ef9..9fe6cd7 100644
--- a/clients/window.c
+++ b/clients/window.c
@@ -244,7 +244,7 @@
 	struct xdg_surface *xdg_surface;
 	struct xdg_popup *xdg_popup;
 
-	struct window *transient_for;
+	struct window *parent;
 
 	struct window_frame *frame;
 
@@ -3912,19 +3912,19 @@
 };
 
 static void
-window_sync_transient_for(struct window *window)
+window_sync_parent(struct window *window)
 {
 	struct wl_surface *parent_surface;
 
 	if (!window->xdg_surface)
 		return;
 
-	if (window->transient_for)
-		parent_surface = window->transient_for->main_surface->surface;
+	if (window->parent)
+		parent_surface = window->parent->main_surface->surface;
 	else
 		parent_surface = NULL;
 
-	xdg_surface_set_transient_for(window->xdg_surface, parent_surface);
+	xdg_surface_set_parent(window->xdg_surface, parent_surface);
 }
 
 static void
@@ -3955,7 +3955,7 @@
 
 	if (!window->custom) {
 		if (window->xdg_surface) {
-			window_sync_transient_for(window);
+			window_sync_parent(window);
 			window_sync_margin(window);
 		}
 	}
@@ -4428,17 +4428,17 @@
 }
 
 void
-window_set_transient_for(struct window *window,
-			 struct window *parent_window)
+window_set_parent(struct window *window,
+		  struct window *parent_window)
 {
-	window->transient_for = parent_window;
-	window_sync_transient_for(window);
+	window->parent = parent_window;
+	window_sync_parent(window);
 }
 
 struct window *
-window_get_transient_for(struct window *window)
+window_get_parent(struct window *window)
 {
-	return window->transient_for;
+	return window->parent;
 }
 
 static void