filter: add filter constraint model

Add a model that keeps track of a list of filter constraint and makes
them accessible from Qt. Sadly, this is mostly repetitive boiler-plate
code, but this is due to Qt's model/view-API, which is a perfect example
of how *not* to design a reasonable modern API.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

qt-models/filterconstraintmodel.h

@@ -0,0 +1,58 @@
+// SPDX-License-Identifier: GPL-2.0
+#ifndef FILTERCONSTRAINTMODEL_H
+#define FILTERCONSTRAINTMODEL_H
+
+#include "core/filterconstraint.h"
+#include <QAbstractTableModel>
+#include <vector>
+
+class FilterConstraintModel : public QAbstractListModel {
+	Q_OBJECT
+public:
+	enum Roles {
+		TYPE_ROLE = Qt::UserRole + 1,	// enum filter_constraint_type cast to int
+		IS_STAR_WIDGET_ROLE,		// represent as a star widget
+		HAS_DATE_WIDGET_ROLE,		// has a date widget
+		HAS_TIME_WIDGET_ROLE,		// has a time widget
+		NUM_DECIMALS_ROLE,		// number of decimal places for numeric data
+		NEGATE_COMBO_ROLE,		// combo box entries for negate
+		STRING_MODE_COMBO_ROLE,		// combo box entries for string mode or empty list if no string mode
+		RANGE_MODE_COMBO_ROLE,		// combo box entries for range mode or empty list if no range mode
+		MULTIPLE_CHOICE_LIST_ROLE,	// list of translated multiple-choice items
+		STRING_MODE_ROLE,		// enum filter_constraint_string_mode_role cast to int
+		RANGE_MODE_ROLE,		// enum filter_constraint_range_mode cast to int
+		TYPE_DISPLAY_ROLE,		// type for display (i.e. translated)
+		NEGATE_DISPLAY_ROLE,		// negate flag for display (i.e. translated)
+		STRING_MODE_DISPLAY_ROLE,	// string mode for display (i.e. translated)
+		RANGE_MODE_DISPLAY_ROLE,	// range mode for display (i.e. translated)
+		NEGATE_INDEX_ROLE,		// negate index in combo box
+		TYPE_INDEX_ROLE,		// type index in combo box
+		STRING_MODE_INDEX_ROLE,		// string mode index in combo box
+		RANGE_MODE_INDEX_ROLE,		// range mode index in combo box
+		UNIT_ROLE,			// unit, if any
+		STRING_ROLE,			// string data
+		INTEGER_FROM_ROLE,
+		INTEGER_TO_ROLE,
+		FLOAT_FROM_ROLE,
+		FLOAT_TO_ROLE,
+		TIMESTAMP_FROM_ROLE,
+		TIMESTAMP_TO_ROLE,
+		TIME_FROM_ROLE,
+		TIME_TO_ROLE,
+		MULTIPLE_CHOICE_ROLE
+	};
+private:
+	QVariant data(const QModelIndex &index, int role) const override;
+	std::vector<filter_constraint> constraints;
+public:
+	using QAbstractListModel::QAbstractListModel;
+	~FilterConstraintModel();
+	void reload(const std::vector<filter_constraint> &);
+	std::vector<filter_constraint> getConstraints() const; // filters out constraints with no user input
+	void addConstraint(filter_constraint_type type);
+	void deleteConstraint(int row);
+	bool setData(const QModelIndex &index, const QVariant &value, int role) override;
+	int rowCount(const QModelIndex &parent) const override;
+};
+
+#endif