add alert to hazeremoval if HQ processing is disabled

[ralfbrown]

Candidate for addressing #18384.

The module's gui_update() function doesn't ever seem to be called, so right now the alert message's visibility is only set when first opening the image in darkroom.

[KarlMagnusLarsson]

HQ processing does not faithfully reproduce darkroom in darktable export result file when hazeremoval module is used.

One example is this case; https://discuss.pixls.us/t/color-management-issue-for-specific-image/46940. Even with HQ processing enabled there are differences between what is rendered and what is exported.

[TurboGit]

@ralfbrown : gui_update() is called when entering darkroom to initialize the module UI indeed you may want to add the same code in gui_changed() which will be called for every slider changed in GUI.