klipper: split filter logic out of `Klipper` class
The Klipper class is too tangled. This tries to split the filter logic to a new class SystemClipboard to prepare for the future porting. SystemClipboard is a wrapper for KSystemClipboard, with a few logic to filter out invalid clipboard events and lock guards to filter out internal events.
All lock guards are moved to the new class.
Edited by Fushan Wen