Paid Plugins + Scripts Default Scene Manager

• Fixed null error during initialization if plugin UI wasn't opened yet, causing the default scene not to be loaded on VAM start - hopefully no other breaking bugs
• Changed the layout of the scene load action form a bit to be more intuitive. Selecting a scene file that doesn't yet have an action is a way to reset the form to create a new action
• Automatically adds a scene load action for Keybindings for the scene set as user default if an action with the same scene path or name doesn't exist already