front: Remove obsolete error log path from UserPrefs.
front: Make ConfigFile collections final to eliminate null check.
front: Only allow reloading of same ConfigFile.

Allowing a ConfigFile to change its backing file after construction is
not a needed feature and adds complexity to the mental model.  Its main
use is for reloading the file from disk to revert unsaved changes,
which shouldn't require the user to know the original filename anyhow.
front: Replace "sectionless" name with constant in ConfigFile.
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.