Use ICU for AutoWrappingText

Rewrite the word-wrapping code for explanatory labels and other texts
with ICU's BreakIterator. Don't use wxTextWrapper, because its
implementation is naive and only works for European languages.

This fixes word-wrapping for Japanese and Chinese localizations (no more
super-wide preferences windows) and improves it for European languages
by choosing better break points.
Don't override Last-Translator with empty value

If the user didn't provide their name+email for use in the header, don't
overwrite the existing header with bogus information and leave the
previous value in.
Don't bug for email address

Don't ask the user to fill their email address. While it intention
always was only to have properly filled in Last-Translator, it _is_ a
bit spammy-feeling and most users, who translates files for their
internal use, don't really need it. Those who do want preserved in the
file can easily set their info in Preferences.
Designate Poedit as MO editor in Info.plist

It saves MO files, which is considered an editor by OS X sandbox;
NSIsRelatedItemType doesn't work with mere viewers.
Add support for extracting from individual files

Closes #131.
Fix msgfmt invocation quoting in Poedit.Targets

Visual Studio build failed when done from a directory with a space in
it, because the <Exec> elements didn't quote its arguments. Add proper
quoting.

Closes #131.