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/diagramcontext.cpp

@@ -42,6 +42,7 @@ void DiagramContext::add(DiagramContext other)
 */
 void DiagramContext::remove(const QString &key) {
 	m_content.remove(key);
+	m_content_show.remove(key);
 }
 
 /**