Add FileSelection Widget

[AlexKnauth]

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

Implements the FileSelection Widget as a button in the settings Gui that opens a file dialog when pressed. The user can select a file, and the WASI-compatible path to that file gets stored in the settings map.

This code won't fully work until after the livesplit-core dependency has been updated, but to help with testing I have this branch to point the dependency at my livesplit-core PR branch: https://github.com/AlexKnauth/asr-debugger/commits/file-selection-demo

[AlexKnauth]

The egui_file dependency changed, so I think I need to replace my use of the FileDialog method filter, with either show_files_filter or filename_filter. Depending on which of those I use, I'll need to replace my use of the Filter type with either Filter<PathBuf> or Filter<String>.