Fix for layout jitter in the Column Selector

[texodus]

This is a CSS cleanup/refactor of @finos/perspective-viewer's Column Selector widget, which aims to eliminate "jitter" in the widget's dimensions and layout as columns are activated or the container's size is changed. The responsive logic which divides this column into "active column list", "inactive column list" and "new column widget" sections does a pretty good job of intelligently using the available space, but has bugs. In this Before GIF, notice that

• The "New Column" dialog moves down when opened.
• When the first column is deselected, the entire panel becomes slightly wider.
• .. and also the "New Column" editor drops to the bottom of the container (off camera).
• As each column is deselected, the "deselected" columns list creeps upwards by ~1-2px.
• When closed, the "New Column" dialog is in a different place in relation to the bottom of the previous panel than when there were no columns deselected.

After GIF:

In this Before GIF, when a column is dragged-over a column-accepting input, the input shrinks and drags the rest of the flex container with it.

After GIF:

In this Before GIF, when the "Column Name" column is dragged into the Active Column list, the list expands by 5px forcing the datagrid to the right:

After GIF:

[sc1f]

Pulled down locally - looks good!