Show warning dialog when VPK file name is not pakXX_dir.vpk

[RedMser]

You basically never want to save a VPK like this, so discourage users from doing so. Dialog could have choices of "rename" which reopens the save dialog in same dir, or "keep anyway" which proceeds as usual.

TODO: research mod managers, maybe some allow custom named VPKs? In that case, remove the strict filter from the LoadDialog instead.

[RedMser]

Low priority since default file name is now a valid name.