Allow to remove every item

Removing every Top Menu item will raise a `Unable to update settings.` error.  
If `$items` is empty, we should consider it as a user request to clear the Top Menu.
Merge pull request #3 from zessx/patch-2

[*] MO : Blocktopmenu allow to remove every item in menu
// small fix
[-] BO : Fix PSCFV-12322 www become http://www
[*] FO : add thumbnails for all categories
// Added Readme.md
// Added Readme.md
[-] MO : blocktopmenu - Proper multishop handling #PSCSX-2426
[-] MO : improved/fixed cache id in blocktopmenu
// w3c
//Version update
Merge branch 'dev'
// blocktopmenu.php: punctuation
Merge pull request #7 from AlexEven/patch-1

// blocktopmenu.php: punctuation
//Version update
Merge branch 'dev'
// blocktopmenu.php: typo
Merge pull request #8 from AlexEven/patch-1

// blocktopmenu.php: typo
//Version update
Merge branch 'dev'