Add command to create a directory in a new category "files"

Use the alt + (left click) is tree view to create a directory and
shift + (middle click) to remove a project directory.

data/core/command.lua

@@ -59,7 +59,7 @@ end
 
 
 function command.add_defaults()
-  local reg = { "core", "root", "command", "doc", "findreplace" }
+  local reg = { "core", "root", "command", "doc", "findreplace", "files" }
   for _, name in ipairs(reg) do
     require("core.commands." .. name)
   end