UI: Use "enabled" instead of "disabled" variable name

Avoids double-negative naming.

UI/undo-stack-obs.cpp

@@ -64,7 +64,7 @@ void undo_stack::add_action(const QString &name, undo_redo_cb undo,
 
 void undo_stack::undo()
 {
-	if (undo_items.size() == 0 || disabled)
+	if (undo_items.size() == 0 || !enabled)
 		return;
 
 	last_is_repeatable = false;
@@ -88,7 +88,7 @@ void undo_stack::undo()
 
 void undo_stack::redo()
 {
-	if (redo_items.size() == 0 || disabled)
+	if (redo_items.size() == 0 || !enabled)
 		return;
 
 	last_is_repeatable = false;
@@ -112,7 +112,7 @@ void undo_stack::redo()
 
 void undo_stack::enable()
 {
-	disabled = false;
+	enabled = true;
 	last_is_repeatable = false;
 
 	ui->actionMainUndo->setDisabled(false);
@@ -122,7 +122,7 @@ void undo_stack::enable()
 
 void undo_stack::disable()
 {
-	disabled = true;
+	enabled = false;
 	last_is_repeatable = false;
 
 	ui->actionMainUndo->setDisabled(true);

UI/undo-stack-obs.hpp

@@ -29,7 +29,7 @@ class undo_stack : public QObject {
 	ui_ptr ui;
 	std::deque<undo_redo_t> undo_items;
 	std::deque<undo_redo_t> redo_items;
-	bool disabled = false;
+	bool enabled = true;
 	bool last_is_repeatable = false;
 
 	QTimer repeat_reset_timer;