LineEdit no longer actually handles motion, it simply indicates which
action was taken to cause it to lose focus (e.g. which key was pressed).
It's up to the client code to determine what this means. This allows
significant consolidation of keyboard-handling logic, and the
implementation of more extensive keyboard navigation features.
New keyboard shortcuts include a tab counter to implement auto-return,
plus Ctrl->Arrow, End, Home, Ctrl-End, and Ctrl-Home, matching the
behavior of OpenOffice, LibreOffice, etc.
Block selection via keyboard has also been added by holding down the
shift key during navigation with the arrow keys (this also works in
combination with the Ctrl modifier for region navigation).
After pull request https://github.com/FreeCAD/FreeCAD/pull/4215 it
doesn't make much sense helping the user to write expressions without
leading '=', as that content will be parsed as a string.
This change adjusts the behaviour in SpreadsheetGui's LineEdit to avoid
popping up the ExpressionCompleter when no leading equal sign is used.
