Provide clear direction on database unlock dialog

[droidmonkey]

Expected Behavior

New users will understand the different options on the unlock dialog, the meaning of the checkboxes, and the definition of "key file".

Current Behavior

User confusion, especially surrounding the checkboxes and key files.

A picture is worth a thousand words:

[notand]

Related request: Improve error message for database unlock dialog

Current Behavior

If the user checks the "Key File" Checkbox in a case in which there is no key file (and if they enter some inappropriate file in the "Key File" field), the following error message is displayed: "Unable to open the database. Wrong key or database file is corrupt."

Expected Behavior

Not exactly sure. Something like "Wrong key or database file is corrupt or Key File method falsely selected." Or: If the software can actually identify that the entered key file is not really a key file, throw an extra error message dedicated just to this specific error.

[notand]

Do key files have a specific format or are otherwise identifiable by the program? If yes, modify the Key File Browsing functionality.

Expected behavior

In the Key File field, the user can only select a file of an appropriate format. Particularly, the user cannot select a .kdbx file.

[phoerious]

A key file can be anything.

[macau23]

"Wrong key or database file is corrupt" is scary.

[droidmonkey]

Fixed by #3287