Be relocatable on Linux.

Instead of relying on build-time POEDIT_PREFIX value to locate the data
at runtime, use runtime-detected location on Linux.  Still falls back to
POEDIT_PREFIX on other Unix platforms.
Non-ugly welcome screen buttons under GTK+.
Use XDG_CONFIG_HOME location on Unix.

Use the now-standard ~/.config location for the configuration. Migrate settings from the previous location if it doesn't exist yet.
OS X: enable grammar checking too.
OS X: disable Smart Quotes and dashes by default.

This is too dangerous to do by default, because it can break markup
without being noticed.  It also breaks Poedit's handling of in-text
quotes, where \" is replaced, as well as the Display Quotes setting.

Fixes #562.
Update Slovak translation.
Update Kirghiz translation.
Update Italian translation.
Merge pull request #53 from fxbenard/stable

Update French translations.
Don't wipe out environment when calling parser tools.

1.6 introduced a bug where the environment (including PATH) was wiped
clean before calling wxExecute().  Don't do this, only add custom
variables to the existing environment.

This probably affected most custom parsers.  jsgettext in particular was
a known fatality.
Update wxWidgets library.

In particular, include the fix for CRCRLF line endings.
Increase version to 1.6.3, update NEWS.