chore : Add message box for advise use hpdi round factor is unsafe

Add a message box to advise user that use a hdpi round factor can cause
strange render according to :
1 - the selected value
2 - the dpi of the screen
3 - Edit the project on another computer and/or screen who don't have
the same parameters as point 1 and 2.

sources/ui/configpage/generalconfigurationpage.h

@@ -51,6 +51,8 @@ class GeneralConfigurationPage : public ConfigPage
 		void on_DiagramEditor_Grid_PointSize_min_sb_valueChanged(int value);
 		void on_ElementEditor_Grid_PointSize_min_sb_valueChanged(int value);
 
+		void on_m_hdpi_round_cb_clicked(bool checked);
+
 	private:
 		void fillLang();