Add FileSelection Widget

[AlexKnauth]

A companion PR to LiveSplit/livesplit-core#748.

Adds a FileSelection type that implements the Widget trait such that an auto-splitter can put it in their settings gui like this:

#[derive(Gui)]
pub struct SettingsGui {
    /// General Settings
    _general_settings: Title,
    /// Select a text file
    #[filter = "*.txt"]
    text_file: FileSelection,
}