Introduce support for specifying variables from X resources. (#2286)

This patch introduces a new 'set_from_resource' config directive which
allows defining a variable by retrieving its value from the X resource
database. This avoids having to configure a color scheme in multiple
files. The directive takes an additional fallback value which is used
in case the resource cannot be found or during config validation where
no X connection is available.

Furthermore, this patch includes the following changes:
- If the same variable is defined twice, we now properly overwrite the
  value of the assignment rather than inserting two variable definitions
  with the same key.
- We now depend on xcb-util-xrm to query the resource.
- Increase the buffer size for variable / resource assignments.

fixes #2130

parser-specs/config.spec

@@ -18,6 +18,7 @@ state INITIAL:
   error ->
   '#'                                      -> IGNORE_LINE
   'set'                                    -> IGNORE_LINE
+  'set_from_resource'                      -> IGNORE_LINE
   bindtype = 'bindsym', 'bindcode', 'bind' -> BINDING
   'bar'                                    -> BARBRACE
   'font'                                   -> FONT