Add directory selection dialog for save / export functions

[d99kris]

Description from #47: Allow invoking the file selection dialog when already in
the export/save view (e/s) to allow selecting the destination folder before
confirming the export/save with Return.

[d99kris]

After some evaluation I have concluded to leave this out of scope for nmail.

It is difficult to provide an intuitive UI for this given the current dialogs used by nmail (enter means going into sub dir).