This PR gets the `SettingsWindow` struct to subscribe to all
`Entity<Project>` events and any future project entities that are
created. When a project emits an event that signals a worktree has been
added or removed, the settings window refetches all settings files it
can find.

This fixes a bug where the settings ui would notice some project
settings that were created or opened after the `SettingsWindow` has been
initialized.

I also renamed `LOCAL` file mask to `PROJECT` to be inline with the
`SettingsFile` naming convention.

Release Notes:

- settings ui: Fix bug where project setting files wouldn't be detected
if they were created or opened after while an active settings window is
open