Added centering of line on page after re-loading file with a known position.
Added `editor.extradescent` option for line spacing (#305).
Fixed replace-in-files when saving backup copy is turned off.
Removed handling of project dropdown in menu as it's no longer needed (#305).
Reorganized menu shortcut conflict handling (#233).

The earlier logic for shortcut conflict handling depended on the global
menu shortcut being always enabled, which prevented some shortcuts from
being disabled when needed. The new logic doesn't have that deficiency.
Updated menu items to stay enabled only when appropriate object has focus.