[lazarus] RFC for control structure

[Michal Bukovjan]

I am currently working a bit on TListBox and would like top discuss, and
if noone is against, to implement the following :

1. Make a subtle change in TControl hierarchy.

I thought about making a TBorderControl, which would implement border
around a control, i.e. implement the property BorderStyle. In Delphi,
each control that has / needs borderstyle implements it itself,
basically the same way. It is because it is given as a parameter in
CreateParams, and not every control supports border around. In GTK,
however, if I want to make a border, I have to wrap the control into
GtkFrame.

So what I want is instead of TControl -> TWinControl -> TCustomListBox,
which implements the border, make it somewhat more reusable, i.e.
TControl -> TWinControl -> TBorderControl -> TListBox and just any other
control which needs that (TListView, TTreeView, TCustomEdit, etc.)

2. Standardize Component property

Make the fComponent a regular property instead just an exposed pointer.
With regards to this, I suggest we introduce another pointer, say
InternalComponent. This is why.
In GTK, a lot of behaviour is accomplished through embedding components
into another. For instance to create a listbox which is bordered and
scrollable, I have to create GtkFrame (border), GtkScrolledWindow
(enables scrolling its children, the listbox) and eventually GtkList.
To reach the GtkList then, I have to cycle through the children of the
GtkFrame, and then children of GtkScrolledWindow, which is slow. So I
would welcome a pointer to the GtkList in question, say
InternalComponent property. The Component property would still point to
the top-level wrapper (GtkFrame), because some messages need this
(resizing, etc.)

Michal Bukovjan