GetStyle => abstract CalculateStyleMask.

- Make naming more clear - it's not getting the style mask, it's calculating what it should be in order to update the mask
- Make it abstract to prevent it being called from the ctor in future

native/Avalonia.Native/src/OSX/PopupImpl.mm

@@ -29,7 +29,7 @@ private:
         [Window setLevel:NSPopUpMenuWindowLevel];
     }
 protected:
-    virtual NSWindowStyleMask GetStyle() override
+    virtual NSWindowStyleMask CalculateStyleMask() override
     {
         return NSWindowStyleMaskBorderless;
     }

native/Avalonia.Native/src/OSX/WindowBaseImpl.h

@@ -105,7 +105,7 @@ BEGIN_INTERFACE_MAP()
     virtual void BringToFront ();
 
 protected:
-    virtual NSWindowStyleMask GetStyle();
+    virtual NSWindowStyleMask CalculateStyleMask() = 0;
     virtual void UpdateStyle();
 
 private:

native/Avalonia.Native/src/OSX/WindowBaseImpl.mm

@@ -35,18 +35,14 @@ WindowBaseImpl::WindowBaseImpl(IAvnWindowBaseEvents *events, IAvnGlContext *gl,
     lastSize = NSSize { 100, 100 };
     lastMaxSize = NSSize { CGFLOAT_MAX, CGFLOAT_MAX};
     lastMinSize = NSSize { 0, 0 };
-
     lastMenu = nullptr;
     
     CreateNSWindow(usePanel);
     
     [Window setContentView:StandardContainer];
-    [Window setStyleMask:NSWindowStyleMaskBorderless];
     [Window setBackingType:NSBackingStoreBuffered];
-
     [Window setContentMinSize:lastMinSize];
     [Window setContentMaxSize:lastMaxSize];
-
     [Window setOpaque:false];
 }
 
@@ -564,12 +560,8 @@ bool WindowBaseImpl::IsModal() {
     return false;
 }
 
-NSWindowStyleMask WindowBaseImpl::GetStyle() {
-    return NSWindowStyleMaskBorderless;
-}
-
 void WindowBaseImpl::UpdateStyle() {
-    [Window setStyleMask:GetStyle()];
+    [Window setStyleMask:CalculateStyleMask()];
 }
 
 void WindowBaseImpl::CleanNSWindow() {

native/Avalonia.Native/src/OSX/WindowImpl.h

@@ -98,7 +98,7 @@ BEGIN_INTERFACE_MAP()
     bool CanBecomeKeyWindow ();
 
 protected:
-    virtual NSWindowStyleMask GetStyle() override;
+    virtual NSWindowStyleMask CalculateStyleMask() override;
     void UpdateStyle () override;
 
 private:

native/Avalonia.Native/src/OSX/WindowImpl.mm

@@ -553,7 +553,7 @@ bool WindowImpl::IsOwned() {
     return _parent != nullptr;
 }
 
-NSWindowStyleMask WindowImpl::GetStyle() {
+NSWindowStyleMask WindowImpl::CalculateStyleMask() {
     unsigned long s = NSWindowStyleMaskBorderless;
     
     if(_actualWindowState == FullScreen)