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.
Merge branch 'tray_icon_unread' of https://github.com/albel727/uTox

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Merge branch 'winxp_icon_fix' of https://github.com/albel727/uTox