[c++options] Restore the ability to set plot sizes in pixels.

Enable and fix the previously untested GtkOptionGtkUIItem::PLOTSIZE.

This has the potentially unfortunate side effect that integer range
options are assumed to be plot sizes. That's correct for now, but
if some report comes along that needs an integer range option for
something else it will have to be differentiated.

libgnucash/engine/gnc-optiondb.cpp

@@ -21,10 +21,12 @@
  *                                                                  *
 \********************************************************************/
 
+#include <cstdint>
 #include <functional>
 #include <string>
 #include <limits>
 #include <sstream>
+#include "gnc-option-uitype.hpp"
 #include "kvp-value.hpp"
 #include "qofbookslots.h"
 #include "guid.hpp"
@@ -768,9 +770,9 @@ gnc_register_number_plot_size_option(GncOptionDB* db,
                                      const char* key, const char* doc_string,
                                      int value)
 {
-    // Pixel values don't make much sense so always use percent.
+//65K is 10x reasonable, but it's a convenient constant.
     GncOption option{GncOptionRangeValue<int>{section, name, key, doc_string,
-                value, 10, 100, 1}};
+            value, 10, UINT16_MAX, 1, GncOptionUIType::PLOT_SIZE}};
     db->register_option(section, std::move(option));
 }