Make Saved Searches/Dashboards more consistent regarding functionality.

_(Note: This issue is WIP, I am still collecting data points for 1. & 2.)_

Currently, Saved Searches & Dashboards differ in terms of functionality. Most noteworthy differences are:

- Dashboards allow tabs, while Saved Searches do not
- Saved Searches can have a global time range/query (+ search filters)/streams selection that is applied to all widgets and which is persisted
- Dashboards can have time range/query (+ search filters)/streams selection for each widget
- Dashboards can override time range/query (but not search filters) for all widgets temporarily, but this is not persisted
- Dashboards can be viewed in "Full Screen" while Saved Searches cannot

1.) These differences require different handling of logic in different places, e.g.:

- When using "Exclude from results"/"Add to query" we need to decide if we add the generated query to the query (Saved Search) or to the global override (Dashboard)
- TBC

2.) We have run into a couple of issues due to these inconsistencies:

- https://github.com/Graylog2/graylog-plugin-enterprise/issues/9023
- TBC

The backend does not differentiate between Saved Searches & Dashboards, the feature sets are the same, it is only the frontend imposing the forementioned limitations upon the user. We should discuss which of these still make sense with the knowledge we have today and which of these can be lifted, or if we want to converge both towards each other in general to simplify the functionality for the user.