Custom user conf folder

[DavidLeoni]

Currently, to add new configuration XMLs the user has to put them in the conf/ folder inside the distribution. Using the menu item Options->Edit configuration he can even be tempted to modify original XMLs, possibly introducing unrecoverable errors (btw it doesn't work on Linux).

To improve things, It should be possible for the user to put new configuration files in a folder outside the distribution. Such folder should then be scanned by the system and its configuration shown added to default one in the config dropdown. We could also ease the process of creating such new configuration files with some new menu item in the GUI.

A possible directory for custom xmls could be ~/.config/smatch/conf , as~./config is the freedesktop suggested dir for all configurations