Prepare MaweJS for multifile support

[mkoskim]

At the moment, when displaying indices and editor canvases, the file & sections are fixed:

Indices:

mawejs/src/gui/editor/editor.js

Lines 607 to 644 in 24df71a

	return <VBox style={style} className="TOC">
	<SectionIndex
	doc={doc}
	sectID="storybook"
	name="Storybook"
	side={side}
	indexing={indexing}
	updateIndexing={updateIndexing}
	indexed={indexed}
	words={words}
	setActive={setActive}
	track={track}
	/>
	<SectionIndex
	doc={doc}
	sectID="draft"
	name="Draft"
	side={side}
	indexing={indexing}
	updateIndexing={updateIndexing}
	indexed={indexed}
	words={words}
	setActive={setActive}
	track={track}
	/>
	<SectionIndex
	doc={doc}
	sectID="notes"
	name="Notes"
	side={side}
	indexing={indexing}
	updateIndexing={updateIndexing}
	indexed={indexed}
	words={words}
	setActive={setActive}
	track={track}
	/>
	</VBox>

Editor instances:

mawejs/src/gui/editor/editor.js

Lines 808 to 818 in 24df71a

	<Slate editor={editors.draft} initialValue={doc.draft.acts} onChange={updateDraft}>
	<SlateEditable visible={active === "draft"} className="Sheet Regular" highlight={highlightText}/>
	</Slate>
	<Slate editor={editors.notes} initialValue={doc.notes.acts} onChange={updateNotes}>
	<SlateEditable visible={active === "notes"} className="Sheet Regular" highlight={highlightText}/>
	</Slate>
	<Slate editor={editors.storybook} initialValue={doc.storybook.acts} onChange={updateStorybook}>
	<SlateEditable visible={active === "storybook"} className="Sheet Regular" highlight={highlightText}/>
	</Slate>

What is needed is that the implementation gives us freedom to have as many sections in the index (and with editor instances) as we like. Later, we change this so, that it is possible to load sections from multiple files (and of course save them to corresponding files), to create:

1. Multifile stories: for example, long stories to splitted to parts.

2. Story series: Multiple related stories with separate background files.

3. Collections: Collections of stories.

4. Splitting and merging content: making spin-offs from stories, or merging two stories together.