[Rd] Mouse handling in Rgui console

[Duncan Murdoch]

I've just committed some changes to R-devel to make the mouse handling 
in the R console more consistent with Windows conventions:

- insert vs overwrite mode is now indicated by a vertical line/solid 
block cursor
- the mouse can be used to move the cursor (only within the input line)
- if part of the input line is highlighted and text is entered, it will 
replace the highlighted section.

It's not fully consistent with other Windows programs:  e.g. Ctrl-X 
still does "copy and paste".  I tried to add missing functionality while 
making minimal changes to existing behaviour, but some changes were 
unavoidable.

Input handling in Rgui is complicated by the support for multi-byte 
character sets and double-width characters (e.g. Chinese, Japanese), 
which I can't easily test, so I'd appreciate it if others could take a 
look and let me know of any bugs.  This should show up in Windows builds 
of 2.6.0dev on CRAN of revision 41380 or later.

Duncan Murdoch