3.9.0

* Windows for tabs detached from an existing window inherits the size and state ("maximized" or "fullscreen") from the source window like Firefox does.
* Reduce some edge case problems (like infinite loop, stop working, and so on) around re-layouting and synchronization of the sidebar contents.
* Update indent level and parent information correctly for descendant tabs orphaned by tab detaching via any operation ouside of TST.
* Imitate tab focus behavior about Firefox's `browser.tabs.insertRelatedAfterCurrent` and `browser.tabs.selectOwnerOnClose` more correctly.
* Never load pending tabs after they are moved across windows.
* Fix unexpected fail of drag-and-drop of selected multiple trees after drag-and-drop across windows.
* Open `about:reader` URL bookmarks with their original URL (because addons cannot open `about:reader` URLs.)
* Always expose the "Manage Devices..." item under the "Send Tree to Device" menu [in the tab context menu, if it is running on Fennec](https://github.com/piroor/treestyletab/issues/3174).
* Brush up wording of "other tabs" in Japanese locale.
* Reset configs more safely with the "Reset" in the options page.
* Change the method to keep space to show auto-hidden scrollbar: using `margin` instead of `padding` in `tab-item-substance`.
* Never change coloring of auto-hidden scroll bar.
* Reintroduce support for `@import` rule in user styles.
* Totally reconstruct [API to insert extra contents to TST's sidebar area](https://github.com/piroor/treestyletab/wiki/Extra-Tab-Contents-API). Now we can insert arbitrary UI element above/below all tabs, and track events about keyboard input and focus.
* Respond to a [`get-tree`](https://github.com/piroor/treestyletab/wiki/API-for-other-addons#get-tree-information) request more certainly even if it is requested with a `Tab.id` and from a `tabs.onCreated` listener.
* Add a new log module `sidebar/tst-api-frontend`.
* Update `zh_CN` locale by [NightSharp](https://github.com/NightSharp). Thanks!