config: support merging multiple config files

Add the -m|--merge-config command line option to iterate backwards over
XDG Base Dir paths and read config/theme files multiple times.

For example if both ~/.config/labwc/rc.xml and /etc/xdg/labwc/rc.xml
exist, the latter will be read first and then the former (if
--merge-config is enabled).

When $XDG_CONFIG_HOME is defined, make it replace (not augment)
$HOME/.config. Similarly, make $XDG_CONFIG_DIRS replace /etc/xdg when
defined.

XDG Base Dir Spec does not specify whether or not an application (or a
compositor!) should (a) define that only the file under the most important
base directory should be used, or (b) define rules for merging the
information from the different files.

ref: https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html

In the case of labwc there is a use-case for both positions, just to be
clear, the default behaviour, described by position (a) above, does NOT
change.

This change affects the following config/theme files:
  - rc.xml
  - menu.xml
  - autostart
  - environment
  - themerc
  - themerc-override
  - Theme buttons, for example max.xbm

Instead of caching global config/theme directories, create lists of paths
(e.g.  '/home/foo/.config/labwc/rc.xml', '/etc/xdg/labwc/rc.xml', etc).
This creates more common parsing logic and just reversing the direction
of iteration and breaks early if config-merge is not wanted.

Enable better fallback for themes. For example if a particular theme does
not exist in $HOME/.local/share/themes, it will be searched for in
~/.themes/ and so on. This also applies to theme buttons which now
fallback on an individual basis.

Avoid using stat() in most situations and just go straight to fopen().

Fixes #1406

docs/labwc.1.scd

@@ -45,6 +45,9 @@ the `--exit` and `--reconfigure` options use.
 *-h, --help*
 	Show help message and quit
 
+*-m, --merge-config*
+	Merge user config/theme files in all XDG Base Directories
+
 *-r, --reconfigure*
 	Reload the compositor configuration