Style tweaks for controls in window footers

[eritbh]

• Controls in window footers are now laid out consistently
  • Text inputs and buttons are styled to be the same height when shown inline with each other in footers
  • Footers are laid out with flexbox with consistent padding and gap between buttons
• Text and other inputs in footers get the same border color as buttons to stand out more against the more saturated footer background
• Select dropdown chevron is now an embedded SVG instead of a PNG - smaller, and looks good on high-density screens or higher zoom levels

Examples of the new styles:

[ashen-dawn]

The styles look good in testing, a few questions about your CSS selectors, but either way I'm approving it.

[ashen-dawn]

If this rule is specifically for the form being the only child, we may want to do

Suggested change

	.mod-toolbox-rd .tb-window-footer > form {
	.mod-toolbox-rd .tb-window-footer > form:only-child {

[eritbh]

I don't think this is really necessary tbh - there's no case where a form has
siblings in a footer, and I can't think of a reason we'd need to do that in the
future either. Plus, since these rules set the gap and other flex layout stuff
consistently between the form and the footer itself, I think this should work
pretty much as-is in the case where there is a form with arbitrary siblings,
so there's no need to arbitrarily lock this down.