Fix memory leak / performance degradation

[dsabanin]

On every re-render, update() method was calling this.append(list), that kept adding the rows widgets to the MainPanel over and over again, without ever cleaning up.

You could see the performance degradation when moving back and forth through the log list – the more you moved, the slower it became, to the point of unusability.

Cleaning up the list of children of MainPanel before re-adding the updated rows fixed the issue.

[JakeH]

Very nice.

To add to your solution: Creating a new element each time is also a bit wasteful. Would be better to just use the built-in setContent method of the existing element and just omit the call to remove completely.

const [existing] = this.children.filter(o => o.type === 'element');
const list = existing || blessed.element({ tags: true });
list.setContent(content);