LegacyUI: use IJ1's dialogs for opening and saving

[ctrueden]

This is more consistent with the legacy IJ1 experience. It will also
respect the value of the "Use JFileChooser to open/save" setting,
whereas the previous code always used JFileChooser which is
nigh-unusable on OS X.

See also:
http://fiji.sc/bugzilla/show_bug.cgi?id=780
http://fiji.sc/bugzilla/show_bug.cgi?id=781

[ctrueden]

I'd appreciate a code review before merging this. In particular:

• What should the title for the OpenDialog and SaveDialog be? I don't want to just use "Open" and "Save" without first checking what ImageJ1 does normally—I think we should match that.
• In my quick testing, I'm concerned that either A) this change somehow breaks the legacy layer; or B) there was already something wrong with the legacy layer. Specifically: File > Open does not seem to use SCIFIO anymore, but rather falls back to Bio-Formats (!) for .fake files, which suggests that vanilla ImageJ1 is being used. No time to debug tonight, unfortunately.

[dscho]

A quick git grep 'new OpenDialog' suggests that Open (and likewise Save) are reasonable choices!

[ctrueden]

Regarding the second bullet point, turns out I was an idiot and just had SCIFIO disabled in my testing. whew!

[dscho]

@ctrueden Heh. It's all too easy to get confused, what with the flurry of bug fixes we're performing these days!