[R-gui] Fixes for tcltk focus problems

[Duncan Murdoch]

I've just committed two Windows-specific changes to r-patched and
r-devel:

 - library(tcltk) now preserves the foreground window.   

Previously it would switch focus to the console (in SDI mode) or just
lose it (in MDI mode)

 - In MDI mode, bringToTop() now brings the MDI frame forward before 
bringing forward the requested window within it.  

The second change was an attempt to fix library(tcltk) in MDI mode,
but it didn't handle the focus change in something like

 windows(); library(tcltk)

properly.  (This should leave focus on the graphics window.)  I put a
lower level change into the tcltk initialization code, then decided
that the bringToTop change was good too.

Duncan Murdoch