Namely to the same file in which FontStyle is declared.

This will make it easier to introduce a theme_load.rs file later.

It is safe to remove because it has never been re-exported as public API.

To make it easier to later introduce a 'plist-load' feature similar
to the existing 'yaml-load' feature, but for themes and not syntaxes.

I have confirmed that this change does not affect the public API.

plist brings in several dependencies, most notably the heavy xml-rs dependency.
So if you don't need to parse theme files, it is nice to be able to skip
this dependency.

For example it brings down the number of dependencies to build from 68 to 52
when building examples/synhtml.rs as a regular app. And build time shrinks
proportionally. Try yourself with:

    cp examples/synhtml.rs src/main.rs # temporarily side-step dev-dependencies
    cargo build --no-default-features --features parsing,html,assets,dump-load,regex-onig