#10254 made the Object Explorer collapsible by clicking the current
workspace icon, but left that as the only way to do it. This adds a
shortcut, Ctrl+Alt+B by default, registered as its own
toggle_object_explorer preference so anyone can change it.
Ctrl+Alt+B rather than anything in the Shift+Alt family, which is full: b,
f, o, l, h, q, v, s, n, e, d and g are all taken, b by the existing Object
Explorer shortcut, which keeps its meaning of focusing the tree. Ctrl+Alt+B
is close enough to VS Code's Ctrl+B for the side bar to be guessable, and
keeps a family resemblance to the bracket keys used for tab navigation.
Ctrl+Shift+B would have been the other obvious candidate, but Chrome and
Firefox both use it for the bookmarks bar.
Worth noting for anyone reviewing: on keyboard layouts where AltGr is
Ctrl+Alt, this combination can be swallowed by the layout and produce a
character instead. That is true of any Ctrl+Alt binding, the preference is
user-configurable, and Shift+Ctrl+Alt+B is available if the default turns
out to annoy people.
The shortcut itself only fires an event, since the workspace provider owns
the visibility state, and the provider ignores it in classic layout where
there is no workspace toolbar and the Object Explorer is always shown.
Tested in a browser as well as in the unit tests: from the workspace layout
the key collapses and restores the panel, and the preference is registered
alongside the existing shortcuts.
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