Blender Git Loki

Spreadsheet: new spreadsheet editor

This implements the MVP for the new spreadsheet editor (T85879). The functionality
is still very limited, but it proved to be useful already. A more complete picture
of where we want to go with the new editor can be found in T86279.

Supported features:
* Show point attributes of evaluated meshes (no original data, no other domains,
no other geometry types, yet). Since only meshes are supported right now, the
output of the Point Distribute is not shown, because it is a point cloud.
* Only show data for selected vertices when the mesh is in edit mode.
Different parts of Blender keep track of selection state and original-indices with
varying degrees of success. Therefore, when the selected-only filter is used, the
result might be a bit confusing when using some modifiers or nodes. This will
be improved in the future.
* All data is readonly. Since only evaluated data is displayed currently, it has to
be readonly. However, this is not an inherent limitation of the spreadsheet editor.
In the future editable data will be displayed as well.

Some boilerplate code for the new editor has been committed before in
rB9cb5f0a2282a7a84f7f8636b43a32bdc04b51cd5.

It would be good to let the spreadsheet editor mature for a couple of weeks as part
of the geometry nodes project. Then other modules are invited to show their own data
in the new editor!

Differential Revision: https://developer.blender.org/D10566