Allow multiple user preferences for file chooser directories in various contexts (send message dialog, import/export channels)

[rbeckman-nextgen]

Maybe it's just me, but usually when I'm doing a bunch of testing, the directory I use to import/export channels is always different from the directory I use to open files with when I send a message to a channel. If I import a channel, then send some messages, then import another channel, etc., I'm always having to redirect the file chooser back and forth. It seems like at least those two contexts should have mutually exclusive preference settings.

Imported Issue. Original Details:
Jira Issue Key: MIRTH-3015
Reporter: narupley
Created: 2013-10-18T11:18:09.000-0700