Draft: order-editor: Generic dialog class for setting the order of elements

The code may change a lot! I opened MR to have a place for discussion.

Goals

1. Provide a class to customize the order of elements
2. Replace the current column and toolbar settings dialogs (My code already does this)
3. Replace similar "Сustom actions" plugin dialog
4. In another MR, implement a context menu settings dialog based on a new class

Issues that must be resolved to continue implementation

1. "Custom actions" is a plugin, and it works through the thunarx library, which prevents me from simply replacing the dialog with my own. What could I do about this? Add a dialog to thunarx or maybe a more general version in libxfce4ui? I don't know. What are your suggestions?