Remember previous location when exporting a sequence from an alignment

Export selected sequences dialog doesn't remember the last used to save location parameter.

Scenario:

1. Open an alignment.
2. Click on a sequence.
3. Select "Actions > Export > Export selected sequence(s)".
4. Choose another output folder in the "Export to folder" parameter.
5. Export the sequence.
6. Select "Actions > Export > Export selected sequence(s)" again.
   Expected state: folder selected on step 4 is specified in the "Export to folder" parameter.
   Current state: a default value is used.