Don't show Cmd+M as accel in context menu

Cmd+M can't be used for Edit Comment on OS X, because it's already taken
for minimizing. The main Edit menu omitted it already, but the context
menu still showed the accelerator. Remove it from there as well.
Fix bad spacing in comments editing window
Improve TM results scoring for short queries

Fix the issue with a short query (e.g. "TM") having 100% much in a
longer string that happens to contain the entire query as a substring.
Return more almost-exact TM results

Don't limit the search for perfect phrase hits, allow inexact hits too.
TM: never scale exact match 1.0 score

Don't apply score degradation scaling for imperfect lookup methods if
they happen to find the perfect match. This may happen e.g. if the
source contains too many words filtered out by the Lucene analyzer (e.g.
"the").
Sort results inside TranslationMemory code

Returned sorted results, from best to worst. Lucene normally returns
sorted results, but our internal score adjustments may change the order,
so the resorting is needed. Particularly relevant for callers that only
care about the best result.
Remove maxHits from TM and Suggestions APIs

The value wasn't required to be respected and in practice, setting it to
too low (i.e. 1, the only really interesting case) could cause bad
results from Lucene-based TM, due to our score adjustments: if only one
result was ever returned by Lucene, another result with worse Lucene but
better adjusted score would never show up. This happened in practice
when using the "fill from TM" functionality.
PCH-less compilation fix

Fixes compilation broken by 187878d9.
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.