Export the small icons to R from the one list `Blockr.icons` is built from

[nbenn]

The small icons exist only in JavaScript, as Blockr.icons, so markup built in R cannot use them and packages copy them instead. The thin × alone is drawn four more times in the dock stack: close_icon() in BristolMyersSquibb/blockr.dock#482 (sidebar-server.R), a CSS mask over dockview's tab close button (blockr-dock.css), and in BristolMyersSquibb/blockr.dock#483 both view_icons in R and a copy in view-binding.js. Those copies restate check and sliders too, and they have started to drift: the plus in view_icons is a 1.25px stroke, while blockr.ui's is Bootstrap's filled path.

Suggested: keep one list of the SVG strings in R as the source, export an accessor for markup built in R, and have controls_dep() hand the same list to blockr-ui.js as Blockr.icons, so both sides read one copy without a build step. Settling which plus is right fits in the same change.