Assets: Rename "Mark Asset" operator to "Mark as Asset"

This naming change was agreed on during the asset workshop. See
https://code.blender.org/2021/06/asset-browser-workshop-outcomes/#terms-definitions.

UI: Move Mark/Clear asset out of ID Data in Outliner context menu

They are now always shown in the top-level of the Outliner context menu.
Having them in the ID Data submenu was just a hassle to work with, and
often confusing to users - even if technically correct (the asset status
is part of the ID data).

Part of T82680.

Fix incorrect poll function used for file path dropping

The operator to drop file paths into the File Browser was just checking
if there's an active window. This wasn't really an issue since the
operator was only used as drop-operator for the File Browser. But the
operator would show up in the operator search. Plus, for asset browsing,
we'll also have to check the file browsing mode, so the more specific
poll function will be needed.

Assets: Disable File Browser only operators for asset browsing

These operators shouldn't be available in the Asset Browser.

https://developer.blender.org/T83556

Added a comment to each operator poll assignment to explicitly mention
the intention. That should also remind devs to decide if the operator
should apply for both file & asset browsing when copy & pasting operator
definition code.

Cleanup: Refactor logic of file deletion poll callback

* Early exit instead of complex if-else blocks.
* Avoid iterating over entire file list.
* Use `true`/`false` for boolean values.
* Declare variables in smaller scopes.

Cleanup: Consistent indent style for asset CMakeLists file

Was mixing 4 and 2 space indent in a single file.

Cleanup: Pass asset handle to asset iterator, rather than wrapped file

This iterator was introduced before `AssetHandle` existed, so it was
dealing with the file data directly. Now we want as little code as
possible to deal with the file data, all access should happen via the
`AssetHandle`.

Assets/UI: Sanity check argument for UILayout.template_asset_view()

Was already doing some sanity checks, but wasn't checking if the passed
property actually is a collection property, which is important.

Cleanup: Remove unnecessary code for asset view UI template

From what I can tell there is no reason anymore to do this. The design
has changed since this was added.

Assets/UI: Resolve major asset view UI template limitation

Before this, all asset view templates showing the same asset library
would show the same assets, even if they should show different ID types.
That was a major limitation since the design did forsee that this
template can be put anywhere in the UI to display various sub-sets of
assets.

Initially I did the ID type filtering close to the asset-list reading,
because I wanted to optimize reading so that we would only actually read
asset information from disk of the ID type to be shown. But this will be
quite complex and I'm not sure if I'll get to work on this anytime soon.
So this commit moves the filtering to the template display level solving
this limitation.

Note: This also adds the code to filter by tags, together with the ID
type. But it's not actually used anywhere yet.