Consolidate common X11 XSetWindowAttributes flags

We always specify CWOverrideRedirect. So just assign that to flags
once, instead of assigning it to flags in four different cases.
Enable X11 backing store

In our XCreateWindow call, we always set the backing_store attribute
to 'Always'. But since we never add the CWBackingStore to our value
mask, the backing_store setting always gets set to the default
NotUseful. This means that the backing store is never turned on, and
so when unobscuring the conky window, portions of the display will not
be drawn immediately if conky is blocked doing something else (e.g.
network, disk, the process is stopped), which is really annoying.

To fix this, specify the CWBackingStore flag in our value mask, so the
conky data is always visible, even if conky is blocking on some other
operation.
Merge pull request #71 from adeason/x11-bstore

Enable X11 backing store