Allow to source configuration files from common directories

Now custom configuration files are sourced from the following
directories (in that order):

- /etc/gdb-dashboard/
- $XDG_CONFIG_HOME/gdb-dashboard/ (or ~/.config/gdb-dashboard/)
- ~/Library/Preferences/gdb-dashboard/
- ~/.gdbinit.d/

This provides a better implementation of and closes #238.

Author: cyrus-and

.gdbinit

@@ -29,6 +29,7 @@ python
 # Imports ----------------------------------------------------------------------
 
 import ast
+import itertools
 import math
 import os
 import re
@@ -625,7 +626,17 @@ class Dashboard(gdb.Command):
 
     @staticmethod
     def parse_inits(python):
-        for root, dirs, files in os.walk(os.path.expanduser('~/.gdbinit.d/')):
+        # paths where the .gdbinit.d directory might be
+        search_paths = [
+            '/etc/gdb-dashboard',
+            '{}/gdb-dashboard'.format(os.getenv('XDG_CONFIG_HOME', '~/.config')),
+            '~/Library/Preferences/gdb-dashboard',
+            '~/.gdbinit.d'
+        ]
+        # expand the tilde and walk the paths
+        inits_dirs = (os.walk(os.path.expanduser(path)) for path in search_paths)
+        # process all the init files in order
+        for root, dirs, files in itertools.chain.from_iterable(inits_dirs):
             dirs.sort()
             for init in sorted(files):
                 path = os.path.join(root, init)