Rework how `show_if` controls other attributes and variables

[EthanGrahn]

This turned out to be a larger rework than I had thought.

Overview

The @@show_if attribute is now a PanelContainer. All custom attributes and the property itself will be assigned as children to the PanelContainer. When the expression is true, the container will hide itself which also hides the children. The choice to use PanelContainer is so that it will automatically scale itself to fit all child elements.

@@show_if works as a "delayed attribute" meaning it will wait until the next property is being parsed to be initialized. This allows the previous property, and any supplemental attributes, to be loaded into the tree for access.

Resolves #5

attributes.cfg

To reduce the size of the main plugin script and improve maintainability/readability, I moved the attribute_scenes map information into a config file. The section names in the config file match the attributes available for use. This config file is parsed in the main plugin script and constructs the attributes_scenes map dynamically. Although it's basic right now, this could be extended to allow customizing settings per attribute.

Tables

This change is not necessary for this fix, it was an early refactor when I thought adjustments to the tables would be needed. I split table.gd into separate table scripts that all inherit from a base table. This avoids the large if statements in the original script and allows checking types if necessary. (e.g. if node is MultiArrayTableAttribute)

I'm open to making any changes, just let me know!

[EthanGrahn]

Noting a potential issue that may need another PR - If show_if is on the last variables, I expect it to fail because it won't be able to initialize during the next variable.

[don-tnowe]

Wow, this... seems to have been a lot of work.

However:

• I do not merge large refactors like this. While it might improve code readability for the writer, it means I would need to know how the code works to make changes in the future.
• class_names in plugins are generally advised against since they may overlap with class names in the project, and relying on them has a script compilation error that breaks the plugin on first launch
• show_if still works strange: seems to not affect the bottom-most property with this attribute. However, it does seem to work on tables AND regular props now.

I will consider the implemented refactorings, and would appreciate a pull with only changes to address #5!