* Update PlotItem.py
make update decimate to not unhide curves when items added/removed,
while preserving the Max Traces well behaviour
* Update PlotItem.py
fix typo
* Update PlotItem.py
fix: typo with self as argument
* give better name for the function which handles MaxTraces checkstate change
rename it to _handle_max_traces_toggle
* add doc string to updateDecimation
* add test for PlotItem for external curve visibility control
check if hidden curve would stay hidden when adding or removing other items.
* remove additional empty line between methods