Fix compilation under Xcode 6.1

Author: indragiek

Butter/BTRPopUpButton.m

@@ -29,6 +29,7 @@ @interface BTRPopUpButtonContent : BTRControlContent
 @end
 
 @implementation BTRPopUpButton
+@synthesize menu = _menu;
 
 #pragma mark - Initialization
 

Butter/BTRSecureTextField.m

@@ -42,6 +42,7 @@ @interface BTRSecureTextFieldCell : NSSecureTextFieldCell
 @implementation BTRSecureTextField {
 	BOOL _btrDrawsBackground;
 }
+@synthesize highlighted = _highlighted;
 
 // Because properties declared in protocols are not automatically synthesized.
 @synthesize drawsFocusRing = _drawsFocusRing;

Butter/BTRTextField.m

@@ -42,6 +42,7 @@ @interface BTRTextFieldCell : NSTextFieldCell
 @implementation BTRTextField {
 	BOOL _btrDrawsBackground;
 }
+@synthesize highlighted = _highlighted;
 
 // Because properties declared in protocols are not automatically synthesized.
 @synthesize drawsFocusRing = _drawsFocusRing;

Butter/BTRView.m

@@ -12,6 +12,7 @@
 @implementation BTRView {
 	BOOL drawFlag;
 }
+@synthesize flipped = _flipped;
 
 #pragma mark Properties