TST Bookmarks Subpanel per Piro (piro_or)

Tree Style Tab addon is now a sidebar panel, so you cannot use other sidebar panels in parallel, e.g. bookmarks. (See also the bug 1328776 - Provide ability to show multiple sidebar contents parallelly) So TST 3.1.0 and later has the SubPanel API to embed arbitrary contents below tabs as a "subpanel" from other helper addons.

This addon is a clone of Firefox's bookmarks sidebar panel for TST's subpanel. Due to security reasons or limitations of WebExtensions API itself, there are some restrictions:

* Impossible to open non-regular URLs like `about:config`. (This is a restriction of WebExtensions API.)
* Impossible to drag anything to the bookmarks subpanel from elsewhere (except tabs in TST's sidebar).
* All bookmarks are shown with same icon. (This is a restriction of WebExtensions API. See also: the bug 1315616 - Give extensions access to cached favicon URLs)