Child/Dialog window refactoring [Draft/Spec]

[kekekeks]

As a "post-mortem" of #3833

Child windows:

• are shown on top of the parent (GWL_OWNER, WM_TRANSIENT_FOR hint [NSWindow addChildWindow], wl_surface_set_transient)
• minimized when parent window gets minimized
• can get minimized individually on platforms that support it (i. e. not on OSX)
• restored when parent window gets restored
• when parent window is closed get closed as well
• when parent window is closing also get a Closing event and can prevent their own close as well as the parent window close (works transitively for entire chain). When child window agrees to be closed, it gets closed immediately.

Dialog windows:

• are child windows with all implications of those
• can not be minimized individually, only with the parent
• prevent parent window from getting any input
• get activated instead of the parent window when it's activated

API changes:

• Owner property becomes readonly.
• Show(Window parent) overload gets added, it marks the window as a child of the parent
• ShowDialog(IWindowImpl parent) overload gets removed

Internal implementation notes:

• IWindowImpl::ShowDialog gets removed completely
• IWindowImpl gets new members:

  // Sets the parent window
  void SetTransientFor(IWindowImpl parent);
  // Disables the window by either calling EnableWindow on Win32 or enabling our existing event suppression logic on other backends
  void SetEnabled(bool enable);
  // Needed to activate the appropriate child window on X11 and OSX when disabled parent gets user input
  Action GotInputWhenDisabled {get;set;}

• child list management gets removed from platform backends

[danwalmsley]

win32:

• [x] are shown on top of the parent (GWL_OWNER, WM_TRANSIENT_FOR hint [NSWindow addChildWindow], wl_surface_set_transient)

• [x] minimized when parent window gets minimized

• [x] can get minimized individually on platforms that support it (i. e. not on OSX)

• [x] restored when parent window gets restored

• [x] when parent window is closed get closed as well

• [x] when parent window is closing also get a Closing event and can prevent their own close as well as the parent window close (works transitively for entire chain). When child window agrees to be closed, it gets closed immediately.

Dialog windows:

• [ ] are child windows with all implications of those
• [ ] can not be minimized individually, only with the parent prevent parent window from getting any input
• [ ] get activated instead of the parent window when it's activated