This feature works like the select lasso in other editors. In preview
the user can draw a region they want to select and when a strips origin
is in this lasso region the strip gets selected.
In timeline the user can do the same and the strip gets selected when
the strip is in the lasso or some part of the lasso is in the strip.
The tool can be accessed through in the toolbar or via shortcut.
Pull Request: https://projects.blender.org/blender/blender/pulls/143391
b910e04a2a