Expand game variables for PARTs accessible in themes

[azuravian]

Feature description

Some strings that can be used in themes, such as the path bound to the button content in PART_ButtonInstallDirectory can include game variables. I believe these should be expanded before binding.

For example, in themes that implement the install directory button, many of my locally installed games use the {Name} variable in the string.

When clicking on the button, it expands the variable as part of the function and opens the correct folder, but it is displayed with the variable in the theme (see below).

Screenshots