Add support for nested gfxmenu components

[max-ishere]

Relevant GRUB documentation

https://www.gnu.org/software/grub/manual/grub-dev/grub-dev.html#GUI-Components

The GUI component instances are created by the theme loader in gfxmenu/theme_loader.c when a theme is loaded. Theme files specify statements such as ‘+vbox{ +label { text="Hello" } +label{ text="World" } }’ to add components to the component tree root. By nesting the component creation statements in the theme file, the instantiated components are nested the same way.

TODO

• [ ] Refactor of the parsing code?
• [ ] Add a branch for either parse a component property or new nested component in the parser
• [ ] Split this test into smaller tests for component and global property

Related source files in this repo

lib: parser/theme_txt.rs examples: theme_txt.rs

[max-ishere]

It seems not all components should support nested components, only the container types:

Containers:

• canvas
• hbox
• vbox

The other components should not allow such declarations.

Components (non-container):

• label
• image
• progress_bar
• circular_progress
• list (currently hard coded to be a boot menu list)

It would be nice to slightly adjust the implementation of the parser and have 2 stages (the first one already exists and the second one is currently in development, I haven't pushed it yet and it will likely be in a separate branch for now)

• Tokenizer This one will produce a token tree which is currently stored in a parser::theme_txt::Document.
• I cant find the correct term for this one Its job is to take a token tree and turn it from map<string, string> to Rusty structs that can be rendered. At this stage we can also decide if it's ok to have a nested component. The tokenizer would not care about that.