Add a new flag to open "Configure Custom Actions" as standalone window

I was reading #245 and how to improve Custom Actions discoverability. This new flag should work in the same way that thunar --bulk-rename works, it opens Thunar without a file manager window.

How I think should work:

• thunar --custom-actions -> Opens the Custom Actions window
• thunar -a could be a shorthand
• An entry should be added to Settings Manager -> Personal so every new user knows this option exists, encouraging its usage

Considerations

• Currently, Custom Actions need to be launched from Edit -> Configure Custom Actions
• User Custom Actions is a Thunar plugin
• The context menu should pick up newly added custom actions