* Fix SSHAgent identity removal on database lock
* Refactor storage and manipulation of SSHAgent keys to streamline process with multiple db's
* Clear password field when widget is hidden, resolves#2502
The Database, DatabaseWidget, and DatabaseTabWidget classes share many responsibilities in inconsistent ways resulting in impenetrable and unmaintainable code and a diverse set of bugs and architecture restrictions. This patch reworks the architecture, responsibilities of, and dependencies between these classes.
The core changes are:
* Move loading and saving logic from widgets into the Database class
* Get rid of the DatabaseManagerStruct and move all the information contained in it into the Database
* Let database objects keep track of modifications and dirty/clean state instead of handing this to external widgets
* Move GUI interactions for loading and saving from the DatabaseTabWidget into the DatabaseWidget (resolves#2494 as a side-effect)
* Heavily clean up DatabaseTabWidget and degrade it to a slightly glorified QTabWidget
* Use QSharedPointers for all Database objects
* Remove the modifiedImmediate signal and replace it with a markAsModified() method
* Implement proper tabName() method instead of reading back titles from GUI widgets (resolves#1389 and its duplicates #2146#855)
* Fix unwanted AES-KDF downgrade if database uses Argon2 and has CustomData
* Improve code
This patch is also the first major step towards solving issues #476 and #2322.
