Mouse button 4, 5 registration and binding

[osiris2258]

Mouse buttons 4 and 5, commonly bound to forward and backwards (commonly located as thumb buttons on many mice), are not registered by XnView and cannot be bound in Interface->Mouse. It would be handy to be able to bind these, for example for moving up and down the file tree (ie back and forward similar to how they are used in web browsers) or for other tasks at the user's discretion. Since these buttons are now present on most mice, having the ability to use them would be handy, and would supplement/allow replacement of the already present CTRL+click modifiers for some users.