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)

[droidmonkey]

You copy between databases by holding the CTRL key while dragging. This is standard behavior in file systems. The default action should be move as it is the most obvious action when dragging things between two databases. This does not fix #172