CAM: Improve warning text for improper unit scheme

The previous text suggested to change the scheme in the preferences,
however, the preferences only take effect for new documents. Now, that
we have the "ProjectUnitSystem" in place, the setting for the current
document needs to be changed.

The text is restructured accodring to the UX pattern for warnings:
 - What is the problem
 - How can I fix it
 - What happens if I don't
and includes a link to the wiki for further details.

In addition, the 'US Customary' was removed from the proposed schemes as
suggested during review.

Finally, the button text was decapitalized and aligned with the
style used in other parts of FreeCAD, e.g. the Start page.

src/Mod/CAM/Path/Main/Gui/JobDlg.py

@@ -76,31 +76,46 @@ class JobCreate:
         if Path.Preferences.suppressVelocity():
             return
 
-        minute_based_schemes = map(FreeCAD.Units.listSchemas, [2, 3, 6])
+        # schemas in order of preference -- the first ones get proposed to the user
+        minute_based_schemes = list(map(FreeCAD.Units.listSchemas, [6, 3, 2]))
         if FreeCAD.ActiveDocument.UnitSystem in minute_based_schemes:
             return
 
-        msg = translate(
+        # NB: On macOS the header is ignored as per its UI guidelines.
+        header = translate("CAM_Job", "Warning: Incompatible Unit Schema")
+        info = translate(
             "CAM_Job",
             (
-                "The currently selected unit schema: \n"
-                "     '{}' for this document\n"
-                " Does not use 'minutes' for velocity values. \n"
-                " \n"
-                "CNC machines require feed rate to be expressed in \n"
-                "unit/minute. To ensure correct G-code: \n"
-                "Select a minute-based schema in preferences.\n"
-                "For example:\n"
-                "    'Metric, Small Parts & CNC'\n"
-                "    'US Customary'\n"
-                "    'Imperial Decimal'"
+                "This document uses an improper unit schema "
+                "which can result in dangerous situations and machine crashes!"
             ),
-        ).format(FreeCAD.ActiveDocument.UnitSystem)
-        header = translate("CAM_Job", "Warning")
-        msgbox = QtGui.QMessageBox(QtGui.QMessageBox.Warning, header, msg)
+        )
+        details = translate(
+            "CAM_Job",
+            (
+                "<p>This document's unit schema, '{}', "
+                "expresses velocity in values <i>per second</i>."
+                "\n"
+                "<p>Please change the unit schema in the document properties "
+                "to one that expresses feed rates <i>per minute</i> instead. "
+                "\n"
+                "For example: \n"
+                "<ul>\n"
+                "<li>{}\n"
+                "<li>{}\n"
+                "</ul>\n"
+                "\n"
+                "<p>Keeping the current unit schema can result in dangerous G-code errors. "
+                "For details please refer to the "
+                "<a href='https://wiki.freecad.org/CAM_Workbench#Units'>Units section</a> "
+                "of the CAM Workbench's wiki page."
+            ),
+        ).format(FreeCAD.ActiveDocument.UnitSystem, *minute_based_schemes[:2])
+        msgbox = QtGui.QMessageBox(QtGui.QMessageBox.Warning, header, info)
+        msgbox.setInformativeText(details)
         msgbox.addButton(translate("CAM_Job", "Ok"), QtGui.QMessageBox.AcceptRole)
         dont_show_again_button = msgbox.addButton(
-            translate("CAM_Job", "Don't Show This Anymore"),
+            translate("CAM_Job", "Don't show this warning again"),
             QtGui.QMessageBox.ActionRole,
         )