Rework desktop environment autodetection

Merged Philip Müller requested to merge dm-conf into devel

Created by: Kirek

Add default_desktop_executable and default_desktop_file options in profile.conf Setting both options will bypass displaymanager autodetection and use provided values to configure the displaymanager. If one or both are not set it will use diplaymanager autodetection and configuration. Configure displaymanager.conf for calamares.

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