Change drag/drop between databases to default to COPY

[leroivi]

Fixes #172
This changes makes the drop action from one database to another as a copy action, but inside the same database still a move action. As suggested by @NicoHood, this behavior is similar to file system managers with USB keys, and it is safer as it does not remove any data from a database.

Testing strategy

• Create 2 testing databases
• Fill the first with a group, an inner group, and a password entry to each levels
• Drag an entry to the root of the second database
• Verify it's present in both databases
• Drag a group to the root of the second database
• Verify it's present in both databases
• Drag a password from a group to another
• Verify it has been moved
• Drag a group from a group to another
• Verify it has been moved
• Drag a password from a group to another with Control pushed
• Verify it has been copied
• Drag a password from a database to another with Shift pushed
• Verify it has been moved

Type of change

• ✅ New feature (change that adds functionality)
• ✅ Breaking change (causes existing functionality to change)