menubar: consider XDG_HOME_DIR and XDG_DATA_DIRS

The freedesktop specifications let desktop files be stored in
different directories indicated by the environment variables
XDG_DATA_HOME and XDG_DATA_DIRS.

Only use the default value for these variables if the variables are
not defined.

This is important for systmes like NixOS which does not follow the LFS
and installs files differently.

lib/menubar/menu_gen.lua

@@ -7,6 +7,8 @@
 ---------------------------------------------------------------------------
 
 -- Grab environment
+local gtable = require("gears.table")
+local gfilesystem = require("gears.filesystem")
 local utils = require("menubar.utils")
 local icon_theme = require("menubar.icon_theme")
 local pairs = pairs
@@ -18,14 +20,16 @@ local menu_gen = {}
 
 -- Options section
 
-local data_dir = os.getenv("XDG_DATA_HOME")
-if not data_dir then
-    data_dir = os.getenv("HOME") .. '/.local/share'
+--- Get the path to the directories where XDG menu applications are installed.
+local function get_xdg_menu_dirs()
+    local dirs = gfilesystem.get_xdg_data_dirs()
+    table.insert(dirs, 1, gfilesystem.get_xdg_data_home())
+    return gtable.map(function(dir) return dir .. 'applications/' end, dirs)
 end
 
 --- Specifies all directories where menubar should look for .desktop
 -- files. The search is recursive.
-menu_gen.all_menu_dirs = { data_dir .. '/applications/', '/usr/share/applications/', '/usr/local/share/applications/' }
+menu_gen.all_menu_dirs = get_xdg_menu_dirs()
 
 --- Specify the mapping of .desktop Categories section to the
 -- categories in the menubar. If "use" flag is set to false then any of