Make icon compatible with WinXP

The icon failed to render under WinXP both
as an application icon in explorer and as a tray icon.
Just opened and saved it again in GIMP and that fixed it.
Don't know what exactly was wrong, the only difference
I noticed is 256 -> 16 color depth change. But since
the image consists just of the black color of varying
transparency, I think it's fine.
Change tray icon until focused

This is to make having unread messages more apparent,
especially when the window is minimized to tray.

I whipped up a new acid-trip-like easy-to-notice icon
for that purpose, due to the lack of an official
tray notification icon mockups. Feel free to change it.
Fixed crash when sending then saving inline image.
Attempted fix of transfer of large files from 32bit machines.
Added munreks bootstrap node.
Safer saving, write to tox_save.tmp then rename it to tox_save.
Abstract i18n some more
Extract i18n initialization into ui_i18n.c

This allows be more explicit about language fallback
route, and change it per-language.

Localized strings are now accessed via a function call,
instead of exposing a big string array.

This allows additional bounds check with a visible canary
value, and further abstracts i18n implementation.
Italian translation: update
Add unused.h with UNUSED macro
Fix unused parameter warnings in obviously correct places
Fix unused parameter warnings for userdata in callbacks
Fix unused parameter warnings for Tox*.

We have a single global instance of Tox,
so in many places we don't care.
Mark unused coord/element parameters in UI functions.
Fix unused parameter warning in messages_set_typing()

is_typing state is directly extracted from friend/group,
that corresponds to the MESSAGES ui element.
Fix unused parameter warning for tox_thread()

Not used anywhere, and thread creators actually
just pass NULL there.
Merge pull request #433 from albel727/better-i18n-take-two

Better i18n take two
Merge pull request #436 from Ansa89/italian-translation

Italian translation: update
Rewrite context menus with i18n support

CONTEXTMENU struct is generalized by introducing
ondisplay() callback for fetching menu item strings,
and adding a userdata field.

One out-of-the-box CONTEXT variety is provided.

It treats userdata as an array of UI_STRING_IDs
to be displayed (beware, contextmenu_new() function
doesn't copy the array, so make sure it has proper lifespan).

All previously existed context menus are replaced with it.
Add i18n text for various dropdowns