Adds a remove layer button to controls

[ahmadayubi]

Closes #253

[Malvoz]

There is a problem with the layer labels being cut off by the absolutely positioned button.

Setting a margin-right of 15 or 20px on the <label> could be a quick fix for now, although it may break if a user changes the font-size in their browser settings (since the icon is an HTML entity and not an img/svg). Otherwise flexbox may be a better solution. We should also try to ensure minimum dimensions of 44px (per #215). But I don't think it's worth spending too much time on these things until #263 is fixed since we'd probably end up rewriting that CSS anyways.