Improve UI of the FilterPanel (1)

[gogonzo]

Context: Current filter panel can become quite lengthy is not easy to navigate for app users, especially when the app contains 10+ dataset, locating a new variable to filter can involve a lot of scrolling back and forth.

• [x] https://github.com/insightsengineering/teal.slice/issues/70
• [x] https://github.com/insightsengineering/teal.slice/issues/84
• [ ] https://github.com/insightsengineering/teal.slice/issues/13
• [ ] https://github.com/insightsengineering/teal.slice/issues/14
• [ ] https://github.com/insightsengineering/teal.slice/issues/6
• [x] https://github.com/insightsengineering/teal.slice/issues/41

[nikolas-burkoff]

Duplicate of #46 ?

[kumamiao]

consolidated to separate the 2 issues (this one and #46) for 2 increments