Dropdown: SelectAll in multi mode

[veej]

requirements

• When dropdown is in multi mode, it should be possible to add an "all" option that allows to select all the options at once.
• The "All" option can't be selected along with other options: when "all" is selected, it should be the only selected value (this means that all the previously selected options must be cleared), and the dropdown should have the style of a single-value selection (i.e. All shouldn't be shown as a tag)
• When all is selected, it should be possible to select any other value. When this happens, the new value should replace "all"

misc

{optional: other useful info}