refactor(sidebar): move sub-windows into a table

Maintaining secondary table of window IDs is cumbersome and is prone to
getting out of sync with the true state of the sidebar. In preparation
for removal of winids table move all containers (sub-windows of the
sidebar) into "containers" table.

The change is mostly mechanical rename with following exceptions:

 - Sidebar:reifresh_winids() and other places where the code scanned
   entire Sidebar object looking for tables with specific fields, such
   as "winid", or "mount" needed to be adjusted for the new structure

 - Sidebar:new() and Sidebar:reset() have been adjusted to make better
   use of the new sub-table.