Slider: close via shortcut

[math-GH]

Before:
It made me odd, that the slider cannot be closed via a shortcut (f.e. ESC).

After:
The sliders can be closed via the shortcut that closes the menu.

Additional: Via shortcut (f.e. F1), you can now open the documentation everywhere.

Changes proposed in this pull request:

• sliders open/close via anchor (the same as the menus) (before: sliders worked only with JavaScript)
• shortcuts are available everywhere (before: only in the feed streams)
• HTML: slider closer is now after the slider (same as the menus)

How to test the feature manually:

1. open the sliders (f.e. subscription management -> config a feed or category; or config an extension manager ->config an extension)
2. the slider opens
3. close the slider via "close menus" shortcut (f.e. ESC)

Pull request checklist:

• clear commit messages
• code manually tested