DisplayOptions: "Dec"/"Inc" operations in the function bar

For numeric entries in Display Options, this gives users a
hint of which keys operate on the value, and allows the
value to be decreasable/increasable using the mouse as well.

DisplayOptionsPanel.c

@@ -24,9 +24,14 @@ in the source distribution for its full text.
 
 static const char* const DisplayOptionsFunctions[] = {"      ", "      ", "      ", "      ", "      ", "      ", "      ", "      ", "      ", "Done  ", NULL};
 
+static const char* const DisplayOptionsDecIncFunctions[] = {"Dec   ", "Inc   ", "                                                      ", "Done  ", NULL};
+static const char* const DisplayOptionsDecIncKeys[] = {"- ", "+ ", "  ", "F10", NULL};
+static const int DisplayOptionsDecIncEvents[] = {'-', '+', ERR, KEY_F(10)};
+
 static void DisplayOptionsPanel_delete(Object* object) {
    Panel* super = (Panel*) object;
    DisplayOptionsPanel* this = (DisplayOptionsPanel*) object;
+   FunctionBar_delete(this->decIncBar);
    Panel_done(super);
    free(this);
 }
@@ -73,6 +78,28 @@ static HandlerResult DisplayOptionsPanel_eventHandler(Panel* super, int ch) {
             result = HANDLED;
          }
          break;
+      case KEY_UP:
+      case KEY_DOWN:
+      case KEY_NPAGE:
+      case KEY_PPAGE:
+      case KEY_HOME:
+      case KEY_END:
+         {
+            OptionItem* previous = selected;
+            Panel_onKey(super, ch);
+            selected = (OptionItem*) Panel_getSelected(super);
+            if (previous != selected) {
+               result = HANDLED;
+            }
+         }
+         /* fallthrough */
+      case EVENT_SET_SELECTED:
+         if (OptionItem_kind(selected) == OPTION_ITEM_NUMBER) {
+            super->currentBar = this->decIncBar;
+         } else {
+            Panel_setDefaultBar(super);
+         }
+         break;
    }
 
    if (result == HANDLED) {
@@ -104,6 +131,7 @@ DisplayOptionsPanel* DisplayOptionsPanel_new(Settings* settings, ScreenManager*
    FunctionBar* fuBar = FunctionBar_new(DisplayOptionsFunctions, NULL, NULL);
    Panel_init(super, 1, 1, 1, 1, Class(OptionItem), true, fuBar);
 
+   this->decIncBar = FunctionBar_new(DisplayOptionsDecIncFunctions, DisplayOptionsDecIncKeys, DisplayOptionsDecIncEvents);
    this->settings = settings;
    this->scr = scr;
 

DisplayOptionsPanel.h

@@ -7,6 +7,7 @@ Released under the GNU GPLv2+, see the COPYING file
 in the source distribution for its full text.
 */
 
+#include "FunctionBar.h"
 #include "Panel.h"
 #include "ScreenManager.h"
 #include "Settings.h"
@@ -17,6 +18,7 @@ typedef struct DisplayOptionsPanel_ {
 
    Settings* settings;
    ScreenManager* scr;
+   FunctionBar* decIncBar;
 } DisplayOptionsPanel;
 
 extern const PanelClass DisplayOptionsPanel_class;