feat: Preset structure redesign and new storage path

[tfuxu]

Feature description

To make presets more flexible and to allow more functionality (like user-selectable accents, different preset variants, etc.), we should redesign preset file structure.
The proposal for new structure has been filed in GradienceTeam/Community#75.

Including this change, we should also change presets path, as the current one, /home/USER/.config/presets/ isn't ideal.
My proposal is to change it simply to /home/USER/.config/Gradience/presets/ (It will be shared between normal and devel builds), which not only will make more sense for user, and make potential conflicts with other apps avoidable, but it will also allow to create for example, an export folder for GNOME Shell theme.

Note
We will need to create a migration functions to allow users to seamlessly update to new storage path.

Additional Context

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