Fix bugtracker #296: cross-reference text overlaps element label by default

XRefProperties::fromSettings() read the "xrefpos" QSettings key with no
default value. On a fresh install/project, the key doesn't exist yet, so
settings.value(...).toString() returns an empty string. QMetaEnum::keyToValue("")
returns -1 (invalid), which was then cast directly into m_xref_pos as
Qt::AlignmentFlag(-1) -- garbage, despite the class's own default
constructor documenting the intended default as Qt::AlignBottom.

This explains the reported symptom: dynamically generated cross-reference
text for master/slave-linked elements (e.g. magneto-thermal breaker,
thermal relay NC) rendered at an undefined position and overlapped the
element's own label, making the reference unreadable. The reporter's
manual workaround -- explicitly setting alignment to "Bottom" in Project
Properties > New Folio/Cross Referencing -- side-steps the bug precisely
by writing a valid "AlignBottom" value into QSettings, which fromSettings()
then reads back correctly on subsequent loads.

Fix: supply "AlignBottom" as the fallback default for the QSettings read,
matching the constructor's documented default and the reporter's
functioning workaround.

Verified: clean rebuild, only the intended object file recompiled and
linked successfully. Confirmed via a small standalone QMetaEnum test that
keyToValue("") returns -1/invalid while keyToValue("AlignBottom") returns
64 (== Qt::AlignBottom), reproducing the exact mechanism before the fix and
confirming the corrected default resolves to the intended value.

Not verified: a live before/after visual comparison of the rendered
cross-reference text position on an actual magneto-thermal/thermal-relay
diagram (would require constructing a multi-folio project with linked
master/slave elements and comparing label geometry, which was out of
scope for the time available). Confidence rests on the QMetaEnum
mechanism being unambiguous and the fix being a one-line default-value
correction with no other code path affected.

sources/properties/xrefproperties.cpp

@@ -90,7 +90,7 @@ void XRefProperties::fromSettings(const QSettings &settings,
 	m_slave_label = settings.value(prefix % "slave_label", "(%f-%l%c)").toString();
 
 	QMetaEnum var = QMetaEnum::fromType<Qt::Alignment>();
-	m_xref_pos = Qt::AlignmentFlag(var.keyToValue((settings.value(prefix % "xrefpos").toString()).toStdString().data()));
+	m_xref_pos = Qt::AlignmentFlag(var.keyToValue((settings.value(prefix % "xrefpos", "AlignBottom").toString()).toStdString().data()));
 
 	for (QString key : m_prefix_keys) {
 		m_prefix.insert(key, settings.value(prefix + key % "prefix").toString());