Merge inline and non-inline FRIEND_FILE_IN_NEW
Refactor image types and inline sending

Instead of passing around weirdly casted *voids,
two types for images are introduced.

One is platform-independent flexible
UTOX_PNG_IMAGE struct. Its inner type
corresponds to what lodepng library accepts.

Another is platform-dependent UTOX_NATIVE_IMAGE,
that can be used to actually render it on-screen.

All functions and structures that could benefit
from it now use them. Weird casts are removed.

Another unfortunately-too-entangled-to-separate
change is avoiding copying pngdata only to
prefix it with its own size. This was done only
for outgoing inline transfers.

Resulting mutant was passed around, leading to
horrible (sometimes conditional!) "+4" magic offsets
across the whole code. And the copying code itself
was duplicated on all platforms.

Now the png array is kept untouched, accompanied
everywhere with a simple size_t argument instead.
In the only place where it's not readily possible
(tox_postmessage), a tiny struct is allocated
and passed instead. "+4" offsets are eliminated.

The end result is more clean and generic code
all around, with less magic numbers, casts,
type conversion warnings and memory copies and leaks.
Improve image vars naming a bit
Italian translation: update
Merge branch 'albel727-demystify-inline-transfers'

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Merge pull request #592 from Ansa89/italian-translation

Italian translation: update
Rename global window vars to sane names

Global vars "width", "height", "baseline",
and "maximized" are now prefixed with
"utox_window_".

They, especially width and height, were shadowed
too often, not to mention that short simple names
for global vars is simply not healthy.
Add -Wshadow to Makefile
Fix shadowing of redraw() in edit.c
Fix shadowing of len in messages_draw()
Fix shadowing of x in messages_draw()
Fix shadowing of redraw() in messages_mmove()
Fix shadowing of data in messages_selection()
Fix shadowing of self in log_write()
Fix shadowing of size in load_save()
Fix shadowing of msg in tox_message()
Fix shadowing of p in tox_transfer.c
Fix shadowing of dx/dy in ui.c
Fix shadowing of r in tox_av.h
Fix shadowing of fd global variable in xlib/*