Fixes and improvements on the Dropdown and Modal components

bigbluebutton-html5/imports/ui/components/button/styles.scss

...button-html5/imports/ui/components/dropdown/component.jsx

...tml5/imports/ui/components/dropdown/content/component.jsx

...n-html5/imports/ui/components/dropdown/list/component.jsx

...l5/imports/ui/components/dropdown/list/item/component.jsx

...ports/ui/components/dropdown/list/separator/component.jsx

...ton-html5/imports/ui/components/dropdown/list/styles.scss

...uebutton-html5/imports/ui/components/dropdown/styles.scss

...tml5/imports/ui/components/dropdown/trigger/component.jsx

...l5/imports/ui/components/leave-confirmation/component.jsx

...rts/ui/components/nav-bar/settings-dropdown/component.jsx

...ports/ui/components/nav-bar/settings-dropdown/styles.scss

bigbluebutton-html5/imports/utils/keyCodes.js