Turn 'Edit columns' into a multi-select menu

The standard behaviour of dropdown menus is to allow selection of only one option. However, 'Edit columns' is not single-select functionality, but multi-select (you can select / deselect as many columns as you want). This commit disables the standard dropdown behaviour and turns 'Edit columns' into a multi-select menu.

Signed-off-by: Belen <belenbarrospena@gmail.com>