Don't report empty POT files as corrupted.
Set layout direction correctly from the language.

Because Poedit doesn't use wxLocale, wx doesn't set the direction on its
down.  Do it manually, based on the UI language actually picked by
wxTranslations.
OS X: Fix window-modal sheets that would close too late.

The DoIfCanDiscardCurrentDoc() window-modal sheets didn't hide
themselves immediately, but only after the subsequent modal windows
closed.  This resulted in rather confusing display when two modal
windows were shown at the same time.
Remove the requirement to use --enable-stl build of wxWidgets.

Kinda overestimated Linux distros progress there. Oh well.
Setup Travis CI builds.

Instead of only verifying the translations, as was done until now,
actually run a full Linux build to test building and packaging.

Because Poedit's dependencies are a bit too modern for the ancient
Ubuntu 12.04 LTS used by Travis, rely heavily on a PPA with the deps.
Fix 'make distcheck' problems.