Support Firefox's own password manager

[patwonder]

Password manager support has been among the most requested features for quite some time. I feel like Firefox's password manager is a good starting point.

Relevant MDN documents are:

• Using nsILoginManager: https://developer.mozilla.org/en-US/docs/Mozilla/Tech/XPCOM/Reference/Interface/nsILoginManager/Using_nsILoginManager
• nsILoginManager interface: https://developer.mozilla.org/en-US/docs/Mozilla/Tech/XPCOM/Reference/Interface/nsILoginManager#findLogins%28%29

The actual implementation of nsILoginManager is in resource://gre/modules/LoginManagerContent.jsm. Intuitively, we could do something similar to detect fillable forms in plugin code, while query Firefox for credentials.

There are some issues worth considering:

• Shall we support saving passwords from the IE engine?
• stub for more issues