macosx: make sure that the fullscreen window is the main window (close #6850)

With the previous impementation, VLCMainWindow will stay the main window in fullscreen and is accidently activated when closing utility panels.
(cherry picked from commit f2092e1069a7bd65b7a8a99f4da513d06b352d5a)

modules/gui/macosx/MainWindow.m

@@ -1721,6 +1721,7 @@ static VLCMainWindow *_o_sharedInstance = nil;
         [o_fullscreen_window setFullscreen: YES];
         [o_fullscreen_window setBackgroundColor: [NSColor blackColor]];
         [o_fullscreen_window setCanBecomeKeyWindow: YES];
+        [o_fullscreen_window setCanBecomeMainWindow: YES];
 
         if (![o_videoWindow isVisible] || [o_videoWindow alphaValue] == 0.0)
         {

modules/gui/macosx/misc.h

@@ -60,6 +60,8 @@
 {
     BOOL b_canBecomeKeyWindow;
     BOOL b_isset_canBecomeKeyWindow;
+    BOOL b_canBecomeMainWindow;
+    BOOL b_isset_canBecomeMainWindow;
     BOOL b_isFullscreen;
     NSViewAnimation *animation;
 }

modules/gui/macosx/misc.m

@@ -188,6 +188,7 @@ static NSMutableArray *blackoutWindows = NULL;
     }
     return self;
 }
+
 - (void)setCanBecomeKeyWindow: (BOOL)canBecomeKey
 {
     b_isset_canBecomeKeyWindow = YES;
@@ -202,6 +203,20 @@ static NSMutableArray *blackoutWindows = NULL;
     return [super canBecomeKeyWindow];
 }
 
+- (void)setCanBecomeMainWindow: (BOOL)canBecomeMain
+{
+    b_isset_canBecomeMainWindow = YES;
+    b_canBecomeMainWindow = canBecomeMain;
+}
+
+- (BOOL)canBecomeMainWindow
+{
+    if(b_isset_canBecomeMainWindow)
+        return b_canBecomeMainWindow;
+
+    return [super canBecomeMainWindow];
+}
+
 - (void)closeAndAnimate: (BOOL)animate
 {
     NSInvocation *invoc;