Fix series draw in wrong order

[thanhlmm]

Type of PR: Bugfix

PR checklist:

• Addresses an existing issue: fixes Overlapped line chart show wrong color order when hover #794
• Includes tests
• Documentation update

Overview of change:
Change the way we sort sources

Is there anything you'd like reviewers to focus on?

[thanhlmm]

@timocov https://github.com/tradingview/lightweight-charts/blob/master/src/model/pane.ts#L169
The logic is that new added source will get a lower zOrder (-1, -2,...). It does not feel like normal standard, zIndex in css for examples

[timocov]

@thanhlmm thanks for your work! Merged now.