i3bar: Handle DestroyNotify events

Handle DestroyNotify events by removing the tray client from the tray
client list held in memory.

This change is intended to be part of the i3bar's implementation of the
XEmbed protocol. For more information, see:

<http://standards.freedesktop.org/xembed-spec/xembed-spec-latest.html>

According to the XEmbed protocol specification, this is one way for a
tray client to finish the protocol. After this event is received, i3bar
should have no more interaction with the tray client.
persist root window’s background contents to a pixmap

The commit title is fairly technical, so I’ll try to explain.

Recently, users of GDM3 (I’m sure) and LightDM (I think) have reported
that when switching to a new workspace, the contents of the previous
workspace are still visible. i3bar updates, though, so it is the X11
root window which is not being updated here.

When using GDM3, X11 will be started with -background none, and no
background pixmap or pixel is set. Then, apparently,
gnome-settings-daemon will display a fade animation from whatever is
currently on the window to the destination contents. I think this is to
avoid flickering when logging in, which would occur when just setting a
specific background pixmap or pixel.

So, this commit will, when i3 starts first (not on restarts), copy the
contents of the X11 root window (typicall a grey background, at least on
my machine with GDM3) into a pixmap and set that pixmap as background
pixmap. That way, the content will be preserved and one has a
background, instead of what is perceived as a bug :).

This commit has some chance of breakage, so I’m prepared to revert it
unless we can figure out the issues and roll forward.
Remove references to PATH_MAX macro

Since the macro PATH_MAX is not defined on every system (GNU/Hurd being
one of those who do not define it), we remove all references to this
macro. Instead, we use a buffer of arbitraty size and grow it when
needed to contain paths.
libi3/root_atom_contents: Free xcb reply structures

Free memory allocated during xcb calls.