Support width setting on gl-new-dropdown

Background

New dropdowns adjust the width to the text they contain, this is good in many situations but in some cases selecting a width for them is required.

Issue

When changing the width of dropdowns they should adjust accordingly

setting	screen	issue
wider than text		text is centered along with caret
narrower than text		caret becomes smaller/larger

How to reproduce

Set a width on the toggle that does not match the text contents.

.gl-dropdown-toggle {
    width: 160px;
}