Blender Git Loki

Asset Browser UI: Changes to the sidebar layout

The current layout wasn't great at all, and it was planned to polish this. This
does a first round of improvements, some more tweaking may follow.
* Place name button at the top, with no panel and no leading label.
* Add "Preview" panel, people may not want to see the preview all the time,
it's already visible in the file list.
* Move button to browse for a custom preview to the right of the preview, as
icon-only. We have a similar layout in other places (UI-lists, matcaps).
* Don't make the details panel a sub-panel. Looked weird because the parent
doesn't have a header.
* Add info icon to "No asset selected", looks a bit friendlier.
* Minor cleanups in the UI script.

Based on designs and feedback by William Reynish.