So far the Asset Browser just showed the same menus as the File Browser.
Not all of their entries made sense for the Asset Browser though. I
decided to just give them entirely different classes to avoid confusing
if-else checks everywhere. I think the code duplication this adds is a
minor issue, it's better to keep things seperated clearly IMO.

• View menu: Add "Asset Details" toggle for the sidebar region.
• View menu: Remove recursion sub-menu
• View menu: Remove "File Path" region toggle, which doesn't apply for the Asset Browser.

Minor cleanup: Move class around, remove section heading

Asset Browser: Don't show inapplicable filter and display settings

So far the Asset Browser just showed the same popups and settings as the
File Browser. Not all of them made sense for the Asset Browser though.

• Don't show inapplicable recursion toggle.
• Don't show sorting options, which don't work with assets anyway.
• Don't show the Filter popover, there are currently no applicable items in there.

Just like in D12057, I decided to add separate classes for the Asset
Browser case.

Assets: Improve error message when "Clear Asset" fails

When using "Clear Asset" from the Asset Browser but with an asset
selected that is not stored in the current file, we can show a more
informative error message.

Asset Browser: Proper context menu for assets

Add a context menu dedicated to asset operations to the Asset Browser.
There are two separate context menus to keep things separated well and
avoid confusing if-else logic (similar to D12057 & D12059). There polls
make sure they are displayed for the right contexts only.

Also (to be committed as followup cleanup): Remove now unused special
handling for assets in file delete operator.