refactor(code): Fix "copy text" button in dialogs and read_only_depends_on

[cogk]

• Use BaseControl.get_status instead of using custom code to detect read-only field, which won't work in dialogs.
• Use BaseControl.get_model_value and BaseControl.get_value instead of frappe.model.get_value, which won't work in dialogs.
• Rename method to setup_copy_button, the name used in ControlData.
• Show/hide dynamically based on read_only_depends_on by moving setup_copy_button call to refresh.