Mod i3.xsession.desktop for autostart Ubuntu 14.04

i3 started from lightdm properly, but autostart did not work. The line
"X-LightDM-DesktopName=i3" seems to be responsible to allow autostart.

Tested on a clean install of Ubuntu 14.04.
Don't ELOG ipc EOF

Receiving EOF from a client is not an error, but rather a standard way a
client may disconnect from the IPC. This should rather be logged from
a consumer of the libi3 ipc_recv_message() function as a normal client
disconnect event.

fixes #1252
Use XCB_BUTTON_INDEX defines for mouse buttons
replace remaining printf()s with D?LOG

fixes #1252
delete ipc socket when exiting, cleanup tmpdir

fixes #1253
default config: switch to “DejaVu Sans Mono 8” as default font

As mentioned in the comment, this change is motivated by better support
for unicode, right-to-left rendering and scalability on retina/hidpi
setups.
debian: recommend fonts-dejavu-core after we switched the default