add shortcuts to fold blocks

[daiyam]

Description

This change adds the following shortcuts to fold depending on the desired level:

Shortcuts on Window/Linux:

• foldAll: Control + O
• unfoldAll: Control + Shift + O
• foldLevel1: Alt + 1
• foldLevel2: Alt + 2
• foldLevel3: Alt + 3
• foldLevel4: Alt + 4
• foldLevel5: Alt + 5
• unfoldLevel1: Alt + Shift + 1
• unfoldLevel2: Alt + Shift + 2
• unfoldLevel3: Alt + Shift + 3
• unfoldLevel4: Alt + Shift + 4
• unfoldLevel5: Alt + Shift + 5

Shortcuts on macOS:

• foldAll: Command + O
• unfoldAll: Command + Shift + O
• foldLevel1: Control + 1
• foldLevel2: Control + 2
• foldLevel3: Control + 3
• foldLevel4: Control + 4
• foldLevel5: Control + 5
• unfoldLevel1: Control + Alt + 1
• unfoldLevel2: Control + Alt + 2
• unfoldLevel3: Control + Alt + 3
• unfoldLevel4: Control + Alt + 4
• unfoldLevel5: Control + Alt + 5

Shortcuts screen:

Issue fixed

• [Feature request] Collapsable code snippets #1630

Type of changes

• ⚪ Bug fix (Change that fixed an issue)
• ⚪ Breaking change (Change that can cause existing functionality to change)
• ⚪ Improvement (Change that improves the code. Maybe performance or development improvement)
• 🔘 Feature (Change that adds new functionality)
• ⚪ Documentation change (Change that modifies documentation. Maybe typo fixes)

Checklist:

• 🔘 My code follows the project code style
• ⚪ I have written test for my code and it has been tested
• 🔘 All existing tests have been passed
• 🔘 I have attached a screenshot/video to visualize my change if possible

[pihentagy]

I would prefer other shortcuts, like Ctrl + Shift + + for expand all.
Also, I wouldn't reserve commands to fold till level x, rather have a "fold more" and "fold less" shortcuts a la vim.

[pihentagy]

See http://vimdoc.sourceforge.net/htmldoc/fold.html

[daiyam]

@pihentagy You will be able to change them. Even those aren't the ones I'm using...

[paxcodes]

What's stopping this from being merged?