Dropdown styles update according to new designs

Purpose

Update current dropdown styles according to the latest design specs.

As discussed in this thread we'll create separate styles only for new dropdowns - listbox and disclosure but will keep old styles for the dropdowns implemented with GlDropdown.

The redesign will be done in 3 steps

• 🐾 Create separate stylesheets for the new dropdowns, pick CSS classes so that they do not overlap with the current GlDropdown's classes and there is no specificity battle.

• 🐾 Add overflow scrim. Some ideas here #2179 (closed)

• 🐾 Add dropdown panel tips pointing at the toggle #2180 (closed)