[Feature Request] Easier navigation between multiple notes

**Is your feature request related to a problem? Please describe.**
Difficult to navigate easily between different notes. Right now, if I am on a note:

- In desktop mode, I need to open a new window to the notes navigator, then open up another note. Then I have two separate windows with two notes.
- In browser mode, much the same, but a bit nicer because I can at least see notes as tabs on my browser.

**Describe the solution you'd like**
When I am in a note, I should:
- have one-click access to another note (probably via a tab) I am working on by some sort of inbuilt tabs
- have a side-bar with a tree view of notes so I can open up a new one

**Describe alternatives you've considered**
Instead of using the desktop version, use one browser window dedicated to hackmd. That way I can have the left most tab as my notes navigator, then I can use other tabs for notes. There are several problems with this though:
- Bookmarks bar + url bar take up space. If I hide bookmarks bar it hides it on all my other browser windows too, which is not what I want.
- More memory chewed up by my hungry browser.
- Can't quit browser without quitting hack md.