Bookmarks: Added option to require prefix for search

[Heck-R]

Bookmarks can be hidden under an item with a specified label

E.g.:

• Using the new setting with the default empty value group_item_label = will work exactly as before
  

• Using some value like group_item_label = Bookmarks will exclude the bookmarks from the original search options, and provide a single item instead. The bookmarks can be searched upon selecting the mentioned item
  
  

[Heck-R]

Note

There are also 2 issues to which this change is relevant: #217 and #209

While both are already closed, they were only partially addressed.