KConfigXT migration for activity behaviors

This commit introduces PowerDevil::ActivitySettings, generated
from a newly added PowerDevilActivitySettings.kcfg[c] config spec.

It also introduces PowerDevil::migrateConfig(), currently
migrating only activity behavior config but meant to also
migrate profile config as well later. We need to migrate
config data because the existing config data structure
is not particularly well-suited for access with KConfigXT.

Activity config data is migrated from powermanagementprofilesrc
to powerdevilrc. This prevents config interpretation conflicts
vs. Plasma 5. In Plasma 6, we'll only use powerdevilrc.

migrateConfig() is called in PowerDevil::Core on daemon startup.
KCM startup won't invoke migration, people will have generally
started a Plasma 6 session before using Plasma 6 System Settings.

Boolean values for special behaviors are migrated, but only if
the "special behaviors" mode was selected in the previous config.
If the mode was set to a full activity-specific pr...