Fix compile error on Windows

`alloca.h` doesn't seem to be available there.

Fix broken custom preview-icons & failing script_load_addons test

Merge branch 'master' into asset-browser

Support custom preview images for assets

Assets types that already support previews (objects, collections, images,
textures, materials, brushes, lights, worlds, scenes and screens) now support
custom previews too.
To assign a preview, the Asset Browser sidebar shows a "Load Custom Preview"
button. It opens a File Browser to select an image. Once an image is selected,
it overrides whatever preview was previously set. No reference to the image is
created, it basically copies the image to the data-block preview.

The button looks a bit out-of-place currently, that will be improved later.

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

Merge branch 'asset-metadata' into asset-browser

Add custom property support for assets

I didn't do this earlier because I was afraid it would make reading of asset
data (and asset data only!) difficult. Mainly because custom properties could
store pointers to other data, that would have to be read too then (e.g. an
object). But turns out, we can easily disable custom pointers to other
data-blocks, so reading stays simple :).

Merge branch 'asset-metadata' into asset-browser

Merge branch 'master' into asset-metadata

Merge branch 'master' into asset-browser