AK+Everywhere: Rename String to DeprecatedString

We have a new, improved string type coming up in AK (OOM aware, no null
state), and while it's going to use UTF-8, the name UTF8String is a
mouthful - so let's free up the String name by renaming the existing
class.
Making the old one have an annoying name will hopefully also help with
quick adoption :^)

Userland/Services/WindowServer/WindowManagerServer.ipc

@@ -13,5 +13,5 @@ endpoint WindowManagerServer
     set_window_taskbar_rect(i32 client_id, i32 window_id, Gfx::IntRect rect) =|
     set_applet_area_position(Gfx::IntPoint position) =|
     set_workspace(u32 row, u32 column) =|
-    set_keymap([UTF8] String keymap) =|
+    set_keymap([UTF8] DeprecatedString keymap) =|
 }