[BT] Added file open action to context menu.
[BT] Added "open file" action icon.
[Util] Added Filter() to prelude.
[BT] Refactored indices getting to GetSelectedIndexes().
[BT] Support opening multiple files.
[Util] Better Map in prelude.

Support void-returning functions and support non-default-constructible
arguments.
[BT] Added priorities setting menu.