Moved XRandR initialization into XData.

The initialization code for XRandR, which previously existed in
smallwm.cpp, now resides in XData - it has a better home in XData since
XData handles information related to Xlib (beyond the basic
initialization of the display).

This required two other changes:

- Since XData knows about the XRandR event offset, it can provide that
  data to XEvents directly, rather than XEvents having to get it via its
  constructor.
- To handle the potential failure of XRandR initialization, XData now
  has a copy of the logger.