PD: Add offset / overall length modes for LinearPattern

This commit adds support for two separate modes of defining distance
between elements in PD's Linear Pattern.

1. Overall Length - which works exactly like it works before,
2. Spacing - which allows user to explicitly define distance (offset) between
   features.

src/Mod/PartDesign/Gui/TaskLinearPatternParameters.h

@@ -59,7 +59,9 @@ private Q_SLOTS:
     void onUpdateViewTimer();
     void onDirectionChanged(int num);
     void onCheckReverse(const bool on);
+    void onModeChanged(const int mode);
     void onLength(const double l);
+    void onOffset(const double o);
     void onOccurrences(const uint n);
     void onUpdateView(bool) override;
     void onFeatureDeleted() override;
@@ -72,13 +74,16 @@ protected:
     void clearButtons() override;
     void getDirection(App::DocumentObject*& obj, std::vector<std::string>& sub) const;
     bool getReverse() const;
+    int getMode() const;
     double getLength() const;
+    double getOffset() const;
     unsigned getOccurrences() const;
 
 private:
     void connectSignals();
     void setupUI();
     void updateUI();
+    void adaptVisibilityToMode();
     void kickUpdateViewTimer() const;
 
 private: