Variable Dropdown Menus

[dlarv]

Variables are now selected using an option/dropdown menu, to help mitigate typos. Creating a new variable in the side panel will make said variable accessible by all Set, Conditional, Dialogue, and Fork nodes. Variables can still be accessed using the old {{_}} method in places where there are still text inputs.

ExampleVariableDropdowns.tscn was added to the examples as a test file, as it uses every instance of these dropdowns at least once.