[KSysGuardProcessList] Don't forward keys with modifiers (except shift) to the search field

This breaks shortcuts like Ctrl+A to select all. The same approach is used for invoking KRunner on the desktop.
Shift-only modifier is allowed so uppercase letters can still be input properly.

BUG: 335177
FIXED-IN: 5.6.3

Differential Revision: https://phabricator.kde.org/D1085