Add XRC handler for wxAuiManager and include the existing wxAuiNotebook
handler into it (but notice that wxAuiToolBar handler added by a later #15686
in f269f868d7 remains separate).
Also update the AUI dialog in the sample and stop hardcoding its size in
pixels.
See #13520.
The version doesn't really matter as the only change in 2.5.3.0 compared to
2.3.0.1 seems to be the handling of "\\" which doesn't occur in any of these
files anyhow, but use the latest one just in case anybody takes one of these
files as a starting point.
