Editable counter value in the dock, and fix two bugs in the reset button

Follows @scorpio810's review on PR #626 and three defects found by finally
running the thing rather than only building it.

Replace the "?" button with an editable value field, as asked for. It shows
the current value of the context's counter -- the last part that actually
progresses, i.e. the least significant digit -- and typing a new value and
committing it writes that value back. This is strictly more useful than the
button it replaces: "?" is still reachable by typing it, and any other value
is now reachable too, which was the point of the request.

It also removes a destructive edge the button had: "reset to ?" rewrote
*every* part, so a scheme built as "K" + counter became "?????" and the
configured prefix was gone for good. There is no undo command for
numbering contexts.

Two bugs fixed in the reset path itself:

- The project was never marked modified. addConductorAutoNum() and friends
  are a plain insert into a QMap; they emit nothing and set no dirty flag,
  and the properties dialog that this code was modelled on calls
  setModified(true) separately afterwards. Without it the user resets a
  counter, closes the project, is not asked to save, and the reset is lost.
  Verified before the fix: projectWasModified() stayed false across a click.

- A wrap part was reset to "1". A modulo part cycles over [0, modulus) --
  the PLC addressing that motivated the feature runs %IX0.0..%IX0.31 -- so
  its starting value is 0, not 1.

An empty value field is treated as "no change" rather than as an empty
value, so clearing the box by accident cannot wipe a counter, and the field
is refreshed from the context after every write and whenever the selected
context changes.

sources/autoNum/ui/autonumberingdockwidget.h

@@ -24,6 +24,7 @@
 #include <QDockWidget>
 
 class QComboBox;
+class QLineEdit;
 
 namespace Ui {
 	class AutoNumberingDockWidget;
@@ -57,11 +58,12 @@ class AutoNumberingDockWidget : public QDockWidget
 		void on_m_configure_pb_clicked();
 
 		void on_m_conductor_reset_start_pb_clicked();
-		void on_m_conductor_reset_placeholder_pb_clicked();
 		void on_m_element_reset_start_pb_clicked();
-		void on_m_element_reset_placeholder_pb_clicked();
 		void on_m_folio_reset_start_pb_clicked();
-		void on_m_folio_reset_placeholder_pb_clicked();
+
+		void on_m_conductor_value_le_editingFinished();
+		void on_m_element_value_le_editingFinished();
+		void on_m_folio_value_le_editingFinished();
 
 	signals:
 		void folioAutoNumChanged(QString);
@@ -72,12 +74,23 @@ class AutoNumberingDockWidget : public QDockWidget
 		/**
 			@brief resetAutoNum
 			Reset the numerotation context currently selected in combo_box
-			(for the given category) either to a per-type starting value
-			(to_placeholder = false) or to the literal placeholder "?" on
-			every part (to_placeholder = true). Does nothing if no context
-			is selected.
+			(for the given category) to a per-type starting value. Does
+			nothing if no context is selected.
 		*/
-		void resetAutoNum(QComboBox *combo_box, AutoNumCategory category, bool to_placeholder);
+		void resetAutoNum(QComboBox *combo_box, AutoNumCategory category);
+
+			/// Read/write the numerotation context named in combo_box.
+		NumerotationContext contextFor(QComboBox *combo_box, AutoNumCategory category) const;
+		void storeContext(QComboBox *combo_box, AutoNumCategory category, const NumerotationContext &context);
+
+			/// Index of the counter the value field shows and edits: the last
+			/// part that actually progresses. Returns -1 when the context has
+			/// no such part (e.g. it is only fixed text).
+		static int counterIndex(const NumerotationContext &context);
+
+			/// Refresh a value field from its context, and apply a typed value.
+		void refreshValueField(QComboBox *combo_box, QLineEdit *line_edit, AutoNumCategory category);
+		void applyValueField(QComboBox *combo_box, QLineEdit *line_edit, AutoNumCategory category);
 
 		Ui::AutoNumberingDockWidget *ui;
 		QETProject* m_project = nullptr;