[Feat. Req.] Appends a .rcc when preset saving

[trebmuh]

When saving a preset, the user is asked to enter a filename. Works fine till here.

But it looks that radium-compressor doesn't automatically add a .rcc extension to it.

When loading a preset, the browsing-window restricts itself to showing only .rcc (Radium Compressor Configuration) files.

This can then be very confusing when you want to open back your preset since it gives you the feeling that it hasn't been saved.

Please, append automatically a .rcc extension to the filename entered by the user when saving his preset.