Add database merging (content-level sync)

[keepassium]

When a remote database is edited from multiple devices (possibly simultaneously by different users), there is a risk of sync conflicts. This results in multiple slightly different copies of the same database.

Currently, KeePassium detects such issues on save, but the only options are either to overwrite remote changes, or to save local changes as a different file.

A content-level sync would intelligently merge conflicting changes, keeping everything in the same database, and thus significantly improving multi-user/multi-device usability.