[Lazarus] TTabControl reverted to it's old implementation

[Felipe Monteiro de Carvalho]

Hello,

My attempt to fix TTabControl opened a too big can of worms, so I
decided to revert TTabControl to it's previous implementation.

It is not exactly the same as before, but almost the same. Before
TTabControl was implemented in two classes: TCustomTabControl and
TTabControl, now I joined both into only TTabControl. It is a based in
TCustomControl like before.

There is also a new class: TNewTabControl, which is not registered and
is ment to receive patches to implement the new tab control. This new
tab control might then, when finished, be renamed to TTabControl and
the old control might be renamed to TOldTabControl for example.

This way people can keep using the older version of TTabControl, but
we still retain correct names for almost all classes.

-- 
Felipe Monteiro de Carvalho