[Refactor] Making impossible states impossible

[kyu08]

Define AppState according to the concept of 'Making Impossible States Impossible'.
Ref: https://www.youtube.com/watch?v=IcgmSRJHu_8

fzf-make/src/usecase/fzf_make/app.rs

Lines 67 to 75 in e802b23

	pub struct Model<'a> {
	pub app_state: AppState,
	pub current_pane: CurrentPane,
	pub makefile: Makefile,
	pub search_text_area: TextArea_<'a>,
	pub targets_list_state: ListState,
	pub histories: Option<Histories>, // When home dir could not be found, this is None.
	pub histories_list_state: ListState,
	}