[ Feature Request ] Keep session after focus change

### Description
After lost of focus on the window pane with a markdown file either the tab is closed, or live reload option is lost. It would be nice, if there were possibility of opening single page and then live reload on it.

### Current behaviour
1. Open a new .md file
2. Start a preview
3. A new page opens
4. Change your file
5. Switch between panes (to .md file)
6. A new 'session' has to be started: old page freezes and a new page has to be opened

### Suggested behaviour
In the last point: an old session is kept. Page hot reloads itself and switches between files

### Why?
- Simpler workflow with many markdowns
- Single page, that rules them all
- No more opening windows :)

### General info
| Name | Value |
|------------|-----------------------------|
| Complexity | **High** or Medium |
| Urgent | No |
| OP help | No (at least until 2021.07) |