Handle large number of channels

[RFSH]

The current multi-channel support is working under the assumption that the number of channels is not very large. And therefore, we can simply just feed all the images to OSD at once and display them.

But that might not be the case. We might first investigate how OSD viewer behaves when there are a large number of channels (try 10 and 20). Depending on the finding, we might want to impose a maximum channel number by default.

[RFSH]

To test this, I copied the existing channels of an image. The following are links to each one:

• The original image (4 channels)

• Copying the channel info (10 channels)

• Copying the channel info (20 channels)

Based on the browser that you might try this, it might not be able to handle these urls (because of length limitation.) But for me on Chrome 87 works properly. These are what I found out by looking at the images:

• [x] As expected, the page is slow. So we should introduce some heuristics, that would only allow a certain number of channels to be visible by default. Users have to manually ask osd viewer to show the rest of channels.
• [x] We might need show all/hide all control to quickly switch the visibility of image channels.
• [x] We might need collapse all/open all controls to quickly show/hide the channel controls.