Associate native app with .kdbx files

[luckyrat]

For the rare occasions where someone is trying to work out how to import a kdbx file to their Kee Vault, it would be cool if we could offer a way to push the file directly from the native file management app into the Kee Vault Import process. No doubt this is a platform-specific piece of code so won't be trivial to implement.

Will need to make sure it can be done in a safe way that doesn't cause existing unsaved changes to be lost so probably has to be some sort of cached location for the file first and then notify the user that it's ready for import when they are ready to confirm the operation and then enter the file's password.

For the time being at least, users can just use the existing Import feature to select the file so this isn't a high priority enhancement.