TreeView selection aggregation is too keen

[Geoff Bache]

Multiple select and deselect doesn't record enough.
For example multiple_select test in PyUseCase description.

Original text follows:

When testing for TTT-738, I found that if I record a usecase while I
do a few multi-test selections combined with single-test selections, the
resulting usecase only captures a few of the selections. For example,
when I do (texttest -a csp)

+ Select sudoku/easy with the mouse
+ Use downarrow+shift to extend selection to also include sudoku/medium
and sudoku/hard
+ Use uparrow to only select sudoku/medium
+ Use uparrow to select sudoku/easy
+ Use downarrow to select sudoku/medium again
+ Use mouse+shift to select sudoku/super and sudoku/samurai

the resulting usecase is

set test selection to medium under sudoku
set test selection to easy under sudoku
set test selection to medium under sudoku
set test selection to super under sudoku,samurai under sudoku

which I don't think really reflects the selections I did. What's going
on? What I would have expected is

set test selection to easy under sudoku
set test selection to easy under sudoku, medium under sudoku
set test selection to easy under sudoku, medium under sudoku, hard under
sudoku
set test selection to medium under sudoku
set test selection to easy under sudoku
set test selection to medium under sudoku
set test selection to super under sudoku, samurai under sudoku