Klipper: Split the system tray menu between items and context menu

As suggested a long time ago in the referenced bug and duplicates,
but never implemented for the traditional Klipper.  Makes the
system tray popup smaller, especially if the user has configured
a large number of history entries, and makes the traditional Klipper
work in the same way as the Plasma applet:  left click = history items,
right click = context menu.

Add icons to some actions  
Change text for action "show-on-mouse-pos" to reflect what it now does  
Change "KDE" -> "Plasma" in about text  
Eliminate redundant member variable KlipperPopup::m_nHistoryItems  

BUG:93649  
I18N:  
GUI: