Keep the previous versions of a project when saving it

Saving replaced the project file with nothing kept of what it was: one
bad save -- a fault in QElectroTech, a full disk, a wrong edit saved by
mistake -- and the last good version was gone. AutoCAD keeps a .bak on
every save, KiCad several backups.

Before a project is saved over, the file as it was is now copied to a
folder beside it, "<name>-backups", named by the time that version was
saved ("industrial 2026-10-16 08-30-12.qetz"); only the newest are kept,
5 by default (Settings > General > Projects, 0 for none). A copy is
never a move, so a save that fails leaves the file in place; a copy that
fails only warns, the save goes on. The same bytes as the newest version
kept are not kept twice.

The prompt on opening ("Create a backup copy?"), which writes one dated
copy beside the project and never removes any, is left as it is.

New tst_previousversions (newest kept, same bytes once, same second,
nothing for a new file or a count of 0, a failed copy said); it fails
with the oldest never removed. Checked with the program: two scripted
saves of a .qetz leave the original and the first save in
industrial-backups, the project holds the second. ctest 107/107.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

sources/ui/configpage/generalconfigurationpage.ui

@@ -365,6 +365,29 @@
         </widget>
        </item>
        <item row="7" column="0">
+        <widget class="QLabel" name="m_previous_versions_label">
+         <property name="text">
+          <string>Previous versions kept when saving a project</string>
+         </property>
+         <property name="toolTip">
+          <string>Before a project is saved, the file as it was is copied to a folder beside it, named after the project with "-backups". Only the newest are kept.</string>
+         </property>
+        </widget>
+       </item>
+       <item row="7" column="2">
+        <widget class="QSpinBox" name="m_previous_versions_sb">
+         <property name="toolTip">
+          <string>Before a project is saved, the file as it was is copied to a folder beside it, named after the project with "-backups". Only the newest are kept.</string>
+         </property>
+         <property name="specialValueText">
+          <string>None</string>
+         </property>
+         <property name="maximum">
+          <number>100</number>
+         </property>
+        </widget>
+       </item>
+       <item row="8" column="0">
         <spacer name="verticalSpacer_2">
          <property name="orientation">
           <enum>Qt::Vertical</enum>
@@ -1534,6 +1557,7 @@
   <tabstop>m_use_folio_label</tabstop>
   <tabstop>m_border_0</tabstop>
   <tabstop>m_autosave_sb</tabstop>
+  <tabstop>m_previous_versions_sb</tabstop>
   <tabstop>m_common_elmt_path_cb</tabstop>
   <tabstop>m_custom_elmt_path_cb</tabstop>
   <tabstop>m_custom_tbt_path_cb</tabstop>