Tree View: Multi-select support

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

scripts/presets/keyconfig/keymap_data/blender_default.py

@@ -1043,6 +1043,10 @@ def km_user_interface(_params):
         ("ui.view_scroll", {"type": 'WHEELDOWNMOUSE', "value": 'ANY'}, None),
         ("ui.view_scroll", {"type": 'TRACKPADPAN', "value": 'ANY'}, None),
         ("ui.view_item_select", {"type": 'LEFTMOUSE', "value": 'PRESS'}, None),
+        ("ui.view_item_select", {"type": 'LEFTMOUSE', "value": 'PRESS', "ctrl": True},
+         {"properties": [("extend", True)]}),
+        ("ui.view_item_select", {"type": 'LEFTMOUSE', "value": 'PRESS', "shift": True},
+         {"properties": [("range_select", True)]}),
     ])
 
     return keymap