Add numbering scheme for schedule entries without any number

[carlobeltrame]

Multiple people have requested the ability to give no schedule entry number at all to some schedule entries (e.g. mealtimes).

To acheive this, we could...

• Add another numberingStyle (e.g. '-' or 'none')
• Implement the rendering of the numbering style
• Add a special case to the getNumber function which combines day number and schedule entry number, to make sure the combined number is empty as well
• Offer the new style it in the category settings.

For this to work, there are some places in the frontend and in the PDF rendering requiring a modification:

• On the material list view: The display of the schedule entry or of the multiple schedule entries using a material item currently shows only the activity number on small screens
• Same for the material list excel export
• When adding a single activity to a PDF in the print configurator, the dropdown menu currently displays the schedule entry number in parantheses ()
• On the camp's story overview, an empty schedule entry number breaks the vertical alignment of the category label:
  
• In Print Layout #2, the PDF bookmarks of schedule entries would contain two consecutive spaces if the schedule entry number was empty

All other usages I found should work out of the box with empty schedule entry numbers.