Fix Project properties > OK marking the project modified with no edit

Pressing OK in Projet > Propriétés du projet without changing anything
marked the project modified and rewrote its defaults for new folios.
Besides the Page Num placeholder (fixed separately), four causes:

- Title block custom fields: since #271 the dialog offers the
  template's variables for filling in; those left empty came back as
  part of the context. Drop the ones not in the stored context.
- DiagramContext::remove() left the key's show flag behind, so a
  context with a key removed never equalled one without it.
- Guides: applyConf() marked the project modified whether or not the
  guides changed. Compare them like every other setting.
- Wire dash size and cross-reference offsets: the spin boxes cannot
  show the stored default (dash size 1 under a minimum of 2; offset 0
  under a minimum of 40 shown as "Default"), so the clamped value came
  back. Keep the stored value unless the box shows something else, as
  #1204 did for the wire panel.

Checked in the GUI (with the Page Num fix as well) on grafcet,
schema_indus and Projet_vierge: OK with no edit then Ctrl+S saves the
same file as Ctrl+S alone; master writes dash-size 1 -> 2, a
cross-reference offset 0 -> 40, and on Projet_vierge 22 empty custom
fields. New test for DiagramContext::remove() fails without the fix.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

sources/ui/titleblockpropertieswidget.cpp

@@ -166,6 +166,7 @@ void TitleBlockPropertiesWidget::setProperties(
 	// Show the saved custom values, plus any of the template's custom variables
 	// that aren't defined yet, so the user only fills in the missing ones (#271).
 	DiagramContext context = properties.context;
+	m_context_keys = context.keys();
 	addTemplateVariables(context, index);
 	m_dcw -> setContext(context);
 }
@@ -215,6 +216,14 @@ TitleBlockProperties TitleBlockPropertiesWidget::properties() const
 	}
 
 	prop.context = m_dcw -> context();
+		//The template's custom variables are only offered for filling in
+		//(#271): one left empty is not part of the properties.
+	const QList<QString> keys = prop.context.keys();
+	for (const QString &key : keys) {
+		if (!m_context_keys.contains(key)
+				&& prop.context.value(key).toString().isEmpty())
+			prop.context.remove(key);
+	}
 
 	prop.auto_page_num = ui->auto_page_cb->currentText();