File structure flat/refactor

[realdennis]

So far when we want to find and do some update for component such as the previewer, we need to access src/App/Components/Markdown/Previewer/Preview.js let's flat the structure.

And also initial state (initialText.js) is in really nested folder, feel wordy src/App/Container/Hooks/InitialText.js

Create a thread to discuss about it, currently we don't have the action items, please don't just send the PR.