Support dynamic enabling and disabling of controls

See documentation in README for how this works. This commit
handles all the main controls. Pads are not yet supported.

src/ESPUI.cpp

@@ -783,6 +783,7 @@ void ESPUIClass::updateControl(Control* control, int clientId)
     root["id"] = control->id;
     root["visible"] = control->visible;
     root["color"] = (int)control->color;
+    root["enabled"] = control->enabled;
     if (control->panelStyle != 0)
         root["panelStyle"] = control->panelStyle;
     if (control->elementStyle != 0)
@@ -855,6 +856,16 @@ void ESPUIClass::setPanelWide(uint16_t id, bool wide) {
     }
 }
 
+void ESPUIClass::setEnabled(uint16_t id, bool enabled, int clientId) {
+    Control* control = getControl(id);
+    if (control)
+    {
+        control->enabled = enabled;
+        updateControl(control, clientId);
+    }
+}
+
+
 void ESPUIClass::setVertical(uint16_t id, bool vert) {
     Control* control = getControl(id);
     if (control)
@@ -1108,6 +1119,7 @@ void ESPUIClass::prepareJSONChunk(AsyncWebSocketClient* client, uint16_t startin
         item["value"] = String(control->value);
         item["color"] = (int)control->color;
         item["visible"] = (int)control->visible;
+        item["enabled"] = control->enabled;
         if (control->panelStyle != 0)
             item["panelStyle"] = String(control->panelStyle);
         if (control->elementStyle != 0)

src/ESPUI.h

@@ -142,6 +142,7 @@ public:
     bool visible;
     bool wide;
     bool vertical;
+    bool enabled;
     uint16_t parentControl;
     String panelStyle;
     String elementStyle;
@@ -301,6 +302,8 @@ public:
 
     void setPanelWide(uint16_t id, bool wide);
     void setVertical(uint16_t id, bool vert = true);
+    void setEnabled(uint16_t id, bool enabled = true, int clientId = -1);
+
     void updateVisibility(uint16_t id, bool visibility, int clientId = -1);
 
     // Variables