New QLineEdit (m_potential_le) between Type and Nom: an optional,
symbol-author-chosen grouping identifier shared by terminals that
belong to the same physical terminal within a multi-terminal block.
Stored as TerminalData::m_potential (new field, persisted as the
"potential" XML attribute, empty by default).
Used by relatedPotentialTerminal() in terminal.cpp: when
potential_isolating is enabled on a Terminal-type element, terminals
sharing a non-empty, matching potential value now stay electrically
linked to each other instead of every terminal in the block being
isolated from every other one.
Tooltip on the field for explanation