Add support to select multiple tree view elements (similar to outliner/anim channels)
`Ctrl + LMB` to select+activate element under the mouse
`Shift + LMB` to select all items between active and clicked item.
As of now, only Shape key has support for multi-select. (straightforward to include
other views). `KEYBLOCK_SEL` flag is used for storing selection state.
Pull Request: https://projects.blender.org/blender/blender/pulls/138979
a559fb833c