window: Create palette only when needed

Creating a `DecorationPalete` (e.g. the `KColorScheme` inside) is
relatively expensive. Currently, this is done once a client is mapped,
regardless of whether it actually has a server-side decoration.

This change makes it create the palette only when it is actually used,
e.g. the window indeed has a server-side decoration or when the user
actions menu is invoked.

Signed-off-by: Victoria Fischer <victoria.fischer@mbition.io>