when entering side by side mode, the text editor width should be wider

[`http://blockbuilder.org/rveciana/f0a8ec08d0b63d0cdc6985cc37468b9a#mode=sidebyside`](http://blockbuilder.org/rveciana/f0a8ec08d0b63d0cdc6985cc37468b9a#mode=sidebyside)

specifically, something like

``` javascript
let textEditorWidth = windowWidth - toolbarWidth - chartWidth - borderLineWidth - scrollBarWidth
```

current state, with wide gap between chart area and text editor:

Open the linked blockbuilder example in side-by-side mode and inspect the frontend layout code responsible for the toolbar, chart area, and text editor. Verify the width calculation against the available window space; done means the editor uses the freed space without the current wide gap while the chart and toolbar remain visible.

Written by the indexing model from the issue text.