Hide empty dropdown items and collapse items with single child

[lgarczyn]

Proposed changes

Add "None" to root dropdown

Do not add "None" to AssetDropdownItem, SceneDropdownItem or ClassDropdownItem dropdowns

Do not display Assets, Classes or Scene if Empty

Collapse Asset folders if they have a single child

Collapse resulting path if too long

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactoring (no functional changes, no api changes)
• Code style update (formatting, renaming)
• Build related changes (workflow changes)
• Documentation Update (readme, changelog)
• Other: UI tweaks

What is the current behavior?

What is the new behavior?

## Checklist

• The code compiles
• I have tested that this doesn't break existing code (unless it is an explicit breaking change)
• I have tested that the (new) functionality/fix works as intended

[Thundernerd]

My apologies, I have been quite busy recently and haven't had time to look at this yet. I'll review it soon!