Compatibility with other tab unloaders

[Lej77]

When other tab unloaders want to unload the active tab they will switch away from it and then discard it. When the active tab is changed this triggers the listener to the visibilitychange event in the file auto-tab-discard/data/inject/top.js at line 168. This listener will send a message to the background script and this message causes Firefox to set the discarded property to false.

This pull request fixes this issue by adding a slight delay to the message so that if the tab is being unloaded then the message won't be sent.

Steps to reproduce issue

1. Install Auto Tab Discard.
2. Install Unload Tab.
3. Install an extension that shows if a tab is unloaded. For example Tab Center Redux.
4. Switch to a tab that supports context script. (For example google.)
5. Unload the active tab with the Unload Tab extension. (Use alt+x shortcut.)
6. Observe how the discarded tab look loaded even though it isn't.

Browser: Firefox v60.0.1

[rNeomy]

Looks good to me.

Install an extension that shows if a tab is unloaded

btw, If a tab gets unloaded by ATD, its favicon also alters.

[rNeomy]

just a note; as of the next release you can also ask ATD to perform discarding. It has some advantages like the favicon would be uniform;

code sample:

chrome.runtime.sendMessage('atd-id-here', {
  method: 'discard', 
  query: {} // a query object that is passed to chrome.tabs.query
}, ids => console.log('list of discarded tabs', ids))

[Lej77]

@rNeomy Thanks, I will see about implementing support for it in the next version of my Tab Unloader for Tree Style Tab extension.