Blender Git Loki

Asset Browser: Milestone 1 - Basic, Local Asset Browser

NOTE: This is not ready for a merge yet. Purpose of this patch is to give
reviewers a first look. I plan to address some TODOs marked in code, and then
merge it soon to continue work in master. The patch can go through some proper
review then.

For remaining TODOs and project description, see #asset_browser_milestone_1.

This patch includes:
* "Make Asset" operator, exposed in Outliner and button context menus (the
latter is disabled currently)
* Asset meta-data storage and management
* Custom asset tags
* Reading asset meta-data for separate files (but not more)
* Automatic object previews
* Custmo Asset repository setup via Preferences (has to be .blend files
currently), directories are a ToDo.
* Default repository at ~/assets.blend on Linux/macOS, ~/Documents/assets.blend
on Windows
* Asset Browser editor, internally a sub-editor of the File Browser
* Internal File Browser changes need for asset browsing
* "Current File" repository for the File Browser
* Basic UI for editing asset meta-data
* Dragging object, collection, image and material assets into 3D Views
* Option to only show assets on Append/Link
* Some unfinished code for per repository Asset Catalogs (should likely be
disabled for now)

Differential Revision: https://developer.blender.org/D9633