DRAFT: configure clients via content sizes

This is a proposal on simplifying the API for the configure event and reduce the amount of functions that have to be called by the client inside the configure callback.

Previously, when a configure event is received, the client's callback would be called with a libdecor_configuration that contains, amongst others, the window dimensions and state. Since this type is opaque, the client would need to use the additional functions libdecor_configuration_get_content_size and libdecor_configuration_get_window_state to get the content size and the window state from the configuration and then create and commit a libdecor_state with the new desired content size and the original libdecor_configuration.

In this MR, I propose to simplify this by directly providing the content size and window state to the client's callback as the non-opaque type libdecor_state, so that the client does not have to convert or extract the hidden information. Additionally, since we always have to acknowledge a configure event, we should handle this for the client. Hence, the client only has to amend the provided libdecor_state and does not need to call any other API functions inside its configure callback.

To allow this simplification, the client must not commit the top-level surface on its own during this configure event but must wait until libdecor_frame_toplevel_commit gets called.