Add advanced options to Fill Missing Translations

Add two options when filling missing translations from TM:

1. Only use exact matches
2. Don't mark exact matches as fuzzy

This makes some relatively common use cases (especially for
inexperienced users) more manageable.
Add strings for better file association names

Use just "PO Translation" instead of "PO Translation File" - the latter is redundant and there's usually not enough room to show it.

Only change the localizable strings for now and not Info.plist; do the latter once there's enough translations to ease the transition.
Update POT file for 1.7
Update version to 1.7beta3
Revert "Skip OS X sandboxing code when not sandboxed"

This reverts commit 8f12cf6b16c4bf6ab5399271b6e00d29943b1b44.

Fixes #117.