Implement logic for in-memory handling of sensitive credentials

[pat672]

As a jabref user
I want to store my network proxy password in-memory only
So that my password isn't insecurely stored on the file system

As a jabref user
I want to store my shared database password in-memory only
So that my password isn't insecurely stored on the file system

Requirements:

• Requirement R1 - Storing network proxy password in memory only #47
• Requirement R2 - Storing shared database password in memory only #48
• Requirement R3 - Prompt user for network proxy password on startup #49

A search for other sensitive credentials that need such an option was conducted (tracked in #43) but no such credentials were found

[pat672]

Network Proxy Scenarios

These can be used as base for testing

Scenario 1

1. User goes to preferences
2. User configures proxy
3. User checks the "do not save password to file" box
4. User inputs the password and closes the preferences window
5. User makes operation that requires network connecticivity (what should this be concretely? @jolerus could you please look into what actually requires network connectivity and what is the easiest way to trigger such an operation?)

Expectation:

1. User can successfully perform the network operation
2. Password is not stored anywhere on the system

Scenario 2

1. User goes to preferences
2. User configures proxy, checks the "do not save password to file" box
3. User inputs the password and closes the preferences window
4. User restarts the application
5. User makes operation that requires network connectivity (what should this be concretely?)

Expectation:

1. User is prompted for proxy password

Scenario 3

0. User has configured jabref to not store proxy password
1. User restarts the application and is prompted for the proxy password
2. User submits the password to the prompt
3. User makes operation that requires network connectivity (what should this be concretely?)

Expectation:

1. User can successfully perform the network operation
2. Password is not stored anywhere on the system

Scenario 4

1. User goes to preferences and configures proxy, leaves the "do not save password to file" box unchecked
2. User inputs the password and closes the preferences window
3. User restarts the application
4. User makes operation that requires network connectivity (what should this be concretely?)

Expectation:

1. User is not prompted for the proxy password
2. User can successfully perform the network operation
3. Password is stored somewhere on the system

Scenario 5

0. User has configured proxy : a password has been provided and the "do not save password to file" box is unchecked.
1. User goes to preferences and checks the "do not save password to file" box

Expectation:

1. Existing password is deleted from the system