Add UI_FONT_SIZE const in editor

[CopeFiend]

Description

Adds a unique Style key in Editor to be used over the default used in fyrox-ui. (Not yet implemented)

Related Issue(s)

Related issue: #910 , prerequisite for future changes.

Review Guidance

• No visible change in editor, this is a back-end only change that lays the foundation for the future.
• In editor/lib.rs, added new constant

pub const UI_FONT_SIZE: &'static str = "Editor.UI.Font.Size";

to be used over pub const FONT_SIZE as declared in fyrox-ui/src/style/mod.rs

• Added style update handling in fyrox-ui/src/text.rs
• Added pub fn set_style() in fyrox-ui/formatted_text.rs

Screenshots/GIFs

N/A

Checklist

• My code follows the project's code style guidelines
• I have commented my code, particularly in hard-to-understand areas
• I have updated the documentation accordingly
• My changes don't generate new warnings or errors
• No unsafe code introduced (or if introduced, thoroughly justified and documented)

[CopeFiend]

Split from #911

[CopeFiend]

@mrDIMAS Would like to get some early feedback on the rest of the draft, #911 .

The plan is to split the branch into multiple smaller PRs and merge one by one, just need to make sure the foundations are good.