Implement tabs warmup functionality

[emvaized]

After digging in Firefox source code for a while, I found out that it by default does something called "tab warmup" — when user hovers the tab, it kind of preloads it's contents, to make the switch feel faster if user decides to click on it.

Here's the Firefox Source Docs article with more explanation on this feature. Few words from it:

Tab warming allows the browser to proactively render and upload layers to the compositor for tabs that the user is likely to switch to. The simplest example is when a user’s mouse cursor is hovering over a tab. When this occurs, the async tab switcher is told to put that tab into a warming list, and to set its state to STATE_LOADING, even though the user hasn’t yet clicked on it.

Warming a tab queues up a timer to unload background tabs (if no such timer already exists), which will clear out the warmed tab if the user doesn’t eventually click on it. The unload will occur even if the user continues to hover the tab.

Sidebery doesn't do such warmup, which I checked by setting browser.tabs.remote.logSwitchTiming flag in about:configs and hovering over Sidebery tabs. And default tabs panel prints various warmup logs in browser console on hovering tabs.

This PR adds the same functionality in Sidebery using browser.tabs.warmup method, and an option to toggle it in settings. Default value is false just in case — but probably could be true, as native tab panel does it by default.

[mbnuqw]

That's actually was in my task list, thank you! I think we don't need settings for it (I'll hide it myself), just make it turned on by default.

[mbnuqw]

Issue: #284

[emvaized]

was in my task list

Oh, wow — and I thought I will have to justify including this, as it seemed kind of edgy and unusual :)

[emvaized]

I just figured out that it makes no sense to warmup currently active tab on hover (as it's already active). In such case it would be better to warmup the tab which is about to be activated if user decides to close current tab — or perform the "tab switch" action. However, I'm not sure what method it should use — Tabs.findSuccessorTab(tab).id often returns just 1 for me after browser restart. And I guess, it will anyway won't work with tabFlip() action, as it uses a different approach of checking Sidebery's history of tabs?

[mbnuqw]

Yep, the best we can do here is just if (!tab.active) browser.tabs.warmup(tab.id).

Maybe add warmup on switching tabs with mousewheel, but the time required for implementation is not worth the result (at least in my scenarios, I rarely experience slowdown of tab activation).