Added the capability to reset the password manager

Also some documentation about the password manager.

docs/connectionmanager/passwordmanager.rst

@@ -0,0 +1,35 @@
+================
+Password manager
+================
+
+The connection manager has a build-in password manager which uses encryption
+to savely store passwords. If you want to use it is up to you. Check the Save
+password checkbox and the password manager will be used to safely store the
+password. The first time this happens you will be asked for a password you wish
+to use for the password manager. Make sure to remember this password cause there
+is no way to recover it if you loose it.
+
+When you try to open a connection for which the password was saved the program
+will prompt you for the password managers password. You can choose how long the
+program will keep the password manager unlocked. If you cancel this password
+prompt the program will prompt you for the password of the database connection.
+
+------------------------------
+Resetting the password manager
+------------------------------
+
+Unfortunatly at this time there is no way to change the master password.
+Neither is it possible to completely reset the password manager other then
+removing the `pglabuser.db` file.
+
+--------
+Security
+--------
+
+To make it hard to brute force decode the saved passwords the passwords are not
+simply encrypted with the master password. Instead a keystrengthening algorithm
+is applied to your master password combined with a salt. The exact parameters
+choosen for keystrengthening depends on the speed of your computer. This
+strengthened key will be used only as a basis for encryption. Each password
+saved is associated with a random guid, this 128-bit value is combined with
+your strengthened key to create the key to save the password.