Bugfix: use the global root variable, don’t get the first one (Thanks quaec)

Bugfix: use the global root variable, don’t get the first one (Thanks quaec)

The case of an X11 server having multiple displays is handled correctly by the
code in src/mainx.c. However, due to some functions not being correctly
refactored and still getting the first screen (and also the first root window)
from the XCB connection, i3 was operating on the wrong root window.