[interfacekit] Header issues

Since we switched to our own headers, several applications didn't work 
anymore. Marcus and I have made some changes to get binary 
compatibility:

- all headers by Ulrich Wimboek weren't binary compatible - I've worked 
over StringItem.h (originally included in ListItem.h in BeOS R5), 
replaced ListItem.h with the original (removed the BStringItem class, 
though), completely rewritten ListView.h
- some headers by Frans van Nispen weren't binary compatible: 
Invoker.h, and TextControl.h; in BInvoker, he took a bigtime=5Ft for 
fTimeout instead of a uint32

We haven't checked everything, but the apps we have so far are now no 
longer crashing.
You may want to have a closer look at them, though :-)

Checked are (probably something left out here):
- ListItem.h
- StringItem.h
- ListView.h
- Window.h
- Application.h
- Invoker.h
- TextControl.h
- Rect.h
- Cursor.h

Adios...
   Axel.

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