dconf: support configuring specific user databases

[DDoSolitary]

Description

By default, dconf uses $XDG_CONFIG_HOME/dconf/user as the user database, but this can be changed by specifying user-db:<name> in a profile file and setting the DCONF_PROFILE environment variable to that profile. One may want to use different user databases for different DE/WMs to avoid collision.

Currently the module invokes dconf without touching DCONF_PROFILE, which means that 1) it is unable to configure multiple different user databases, and 2) the behavior of activation script will be affected by the DCONF_PROFILE environment variable when it is invoked, possibly leading to undesired results.

This PR adds a dconf.databases option, so that settings under dconf.databases.<name> will be written to $XDG_CONFIG_HOME/dconf/<name>. The old dconf.settings option is left as-is to avoid breaking compatibility.

Checklist

• Change is backwards compatible.

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Maintainer CC

@rycee