Fix manager's edit dialog to not crash.

Due to changes in wx3, incorrectness of the code demonstrated itself
with asserts and crashes.  Rewrote to make more sense.
Use window-modal dialogs where appropriate on OS X.
Fix memory leak when failing to read a catalog.
Better detection of invalid files in Catalog::Load.
Report error when failing to open file.

Instead of, as previously, showing empty window with no explanation,
report that the file cannot be opened and give some guess at the reason.
Fix manager crash when failing to open a file.
Move nuget packages from win32/ to toplevel.

That's what Visual Studio assumes, anything else is recipe for trouble.
Don't build GNU gettext on Windows from sources.

Fuck this shit.  Building gettext from sources is notoriously fragile on
anything but Linux and especially so in the MinGW environment.  Every
little change may break the build with spurious or downright mysterious
errors.  Heck, it occasionally breaks *without* any changes to the
environment!  Not only does the build takes hours (let's be fair, this
one is MSYS/Cygwin's fault), it keeps being rebuilt all the time.  Even
after HOURS spent tweaking the build to ignore source(!) files that the
build regularly touches or even modifies(!!), I still can't get it to
behave reliably and sanely.  (And don't get me started on building from
git, that's an order of magnitude worse ordeal thanks to the masochistic
contraption called gnulib.  I gave up on that early on.)

No.  In the interest of preserving the remains of my sanity, I will just
pull a prebuilt nuget package on Windows.  That's not as bad as it
sounds, I maintain it myself at
https://github.com/vslavik/gettext-tools-windows, so I have some control
over it (and building gettext semi-manually is much simpler than doing
it as part of fully automated builds, I tell you).

Unfortunately, this means that the Windows version looses one recently
implemented improvement: No gettext translations on Windows.  This
should be simple in theory, because gettext has relocatable build on
Windows and should be able to find the catalogs in non-fixed location -
if it worked.  I probably wouldn't be able to make it encode the output
in some sensible charset (read: UTF-8) anyway.

So there.
Don't use Boost as submodule.

Will be replaced with subtree-merged copy instead.

One would assume that "boost-release" repository hosted by the official
boostpro GitHub account will be cared for.  Alas, that's not the case,
the content is not kept updated (and when somebody else does the
updates, the pull request just sits there for 7 months and counting,
being ignored).
Subtree-merge Boost from vendor/boost.
Show Project-Id-Version in window's titlebar.

It is common to name Gettext catalogs just $LANG.po, so the usual
titlebar - just the file name - is not very descriptive.  Include the
project name if defined to make it clear what file is open in Poedit.