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

include/config_parser.h

@@ -11,6 +11,7 @@
 
 #include <yajl/yajl_gen.h>
 
+SLIST_HEAD(variables_head, Variable);
 extern pid_t config_error_nagbar_pid;
 
 /*