make split_value_list() mostly sane

don't count parentheses, don't nest quotes, don't create empty elements.
the backslash still escapes only quotes (and itself) - $$list() (one of
the main users of this function) is commonly used with (windows) path
lists, so letting it escape anything would make a royal mess.

Change-Id: I29252fbe14fd6d28217450ec41cf8acfb2e30681
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
(cherry picked from qtbase/6c22b9b3e86d1617665f7b81b105c032f43c6d72)
(cherry picked from qtbase/a5c2ab47867a558d6f8f62126fb8e4f2777cfe06)