add pointer-clickable pan/zoom controls

[dave-doty]

Currently the only way to zoom is to use the mouse wheel (or simulate it with two-finger motion on the touchpad).

Add custom controls (see here: http://ariutta.github.io/svg-pan-zoom/demo/custom-controls.html) that can be clicked to pan and zoom. There should be buttons with + and - to zoom in and out, and four arrows that can be clicked to pan.

Ideally, position these directly on the SVG elements. Since they themselves should not be panned and zoomed, we need to ensure that the svg-pan-zoom library doesn't place them under the element that controls the matrix transformation.