Fix missing refresh of Asset View UI template previews after regenerating

Fix crash with "Current File" library and "Reload Scripts"

When a (Python defined) UI-list type is unregistered, we have to make sure no
list still references that type. We do the same for other such UI types.
This didn't happen to be an issue before, since the list type was not accessed
until the list was drawn again (which re-assigns the type).

Fix C-linkage warning on Clang

After macro expansion, there would be C++ code in code with C linkage
(`extern "C"`). Clang warned about this.
Make sure the expanded C++ code always uses C++ linkage. Will commit this to
master soon.

Merge branch 'master' into asset-browser-poselib

Asset View: Always show all poses, avoid double-scrollbar

The asset view so far showed up to 5 items by default. If there were
more assets to be shown than that, it the list would become scrollable.
This would cause a scrollable box inside a scrollable region, which is
not nice for interaction. We can just expand the list so all assets are
in the layout, and the user can scroll the region rather than scrolling
the list.

Also hide the "grip" button so the list can't be resized. If needed we
could allow making the list smaller still, even if that causes double
scrolling again. For now keeping it simple.
Had to add a "no grip" option for that, so I changed the signature a bit
to avoid too many boolean arguments.

And another also: Adds missing return after error check.

Fix compile error when using UIList template flags in C++

C++ has strongly typed enums (which is great!), so a bitwise OR on two
enum values will result in a compiler error. Use our `ENUM_OPERATORS()`
utility to overload the bitwise operations to be valid for this strong
enum type.

UI/Nodes: Improve feedback when adding node fails (e.g. on drag & drop)

This is especially useful when trying to add a node group instance, e.g. via
drag & drop from the Outliner or Asset Browser.
Previously this would just silently fail, with no information why. This is a
source of confusion, e.g. earlier, it took me a moment to realize I was
dragging a node group into itself, which failed of course.
Blender should always try to help the user with useful error messages.

Adds error messages like: "Nesting a node group inside of itself is not
allowed", "Not a compositor node tree", etc.

Adds a disabled hint return argument to node and node tree polling functions.
On error the hint is reported, or could even be shown in advance (e.g. if
checked via an operator poll option).

Differential Revision: https://developer.blender.org/D10422

Reviewed by: Jacques Lucke

Merge branch 'master' into asset-browser-poselib

UI/Nodes: Improve feedback when adding node fails (e.g. on drag & drop)

This is especially useful when trying to add a node group instance, e.g. via
drag & drop from the Outliner or Asset Browser.
Previously this would just silently fail, with no information why. This is a
source of confusion, e.g. earlier, it took me a moment to realize I was
dragging a node group into itself, which failed of course.
Blender should always try to help the user with useful error messages.

Adds error messages like: "Nesting a node group inside of itself is not
allowed", "Not a compositor node tree", etc.

Adds a disabled hint return argument to node and node tree polling functions.
On error the hint is reported, or could even be shown in advance (e.g. if
checked via an operator poll option).

Differential Revision: https://developer.blender.org/D10422

Reviewed by: Jacques Lucke

Use correct add-ons branch for `make update`

Also add TODO comments, hoping they help us not forget changing this
back for the merge to master.