Speaking times widget: Enhance time edit menu

[MSoeb]

Description: In the current state, the three-dot-menu behind a structure level misses some time edit options,which leads to some problems.

• If someone has spoken, the time can't be edit. This results in the worst case scenario, that the list of speakers needs to be created again from the scratch.
• If there is a need to add, remove or reset the time, the menu doesn't provide the edit options.

What should happen:

• Editing total time should always be possible. The 'edit total time' options should always be active/enabled.
• Additionally other edit time options should be added, which should always be active. This include three options:

1. Reset timer: The timer should be resetable to the start time of the set structure level timer. NOT the general default time value for structure levels.
2. Add time option: It should be possible to manually add time to the structure level timer.
3. Remove time option: It should be possible to manually remove time from the structure level timer.

[luisa-beerboom]

I created a backend PR for this: OpenSlides/openslides-backend#2220

It changes the functionality is as follows:

1. Setting initial_time is no longer inhibited by whether or not someone has spoken yet
2. If initial_time is changed, remaining_time is increased/decreased by the same amount

[Elblinator]

I'm sorry, your message was not displayed and I opened another backend Issue due to that
I already closed the issue again