Deprecate option to configure a path to a Lexical release directory rather than an executable launcher

[Blond11516]

Since #37, it's possible to use a custom build of Lexical in one of two ways:

1. Configure a path to a directory containing a start_lexical.sh script
2. Configure a path to an executable launcher

The first option should be deprecated since it's not much easier while being offering no flexibility as to the launcher name.

Two options to inform users that their config is deprecated:

1. Directly in the settings UI. I'm not actually sure this is possible. I think only basic type verifications (int, string, enum, etc.) can be done there. Also most people probably don't go there very often so they might not even see the warning.
2. As a notification warning. This would be far more visible, meaning it would incite most users to migrate their config quickly.

Once support for the functionality is removed, it would also be nice to keep checking for it and give a clear error as a notification in case someone hasn't migrated by then.