LabPlot builds and runs with kf5

[Alexander Semke]

> Here is the back trace : http://pastebin.com/Zh4dkRiV
This crash is because of the qtplugin-related lines in worksheet/interfaces.h 
and worksheet/StandardCurveSymbolFactory.cpp you commented out. We need to 
solve this problem. I'd like to redesign the symbols handling and to get rid 
of this plugin and factory stuff involved there. But this will take some 
time...

-- 
Alexander