front: Add method to ConfigFile for removing sections.

Custom input profiles will be persisted in a single ConfigFile.  Users
need a way to delete their custom profiles (i.e. ConfigSections).

We also make ConfigSection#nextName private because it may be invalid
once a section has been removed.  It's an implementation detail anyhow,
only used internally to identify end of file when reading.
front: Fix broken path for default selected game.

Switching over from PathPreference, I forgot there was some special
sauce in the interpretation of certain characters (~, !).  Kind of ugly
but this is only temporary anyhow.