* TickSliderItem: allowRemove property added
Following #744, this PR suggests the addition of a property
`allowRemove` to `TickSliderItem` and therefore also to
`GradientEditorItem`. It sets the default of whether ticks can
be removed by the user and therefore contemplates `allowAdd`.
I would be interested in other opinions on this design decision,
as #744 suggests to reuse `allowAdd` for the prohibition of
tick removal.
I personally suggest this solution, since it does not change
the effect of the current API.
Fix#744
* customPlot.py: Added markers to example using TickSliderItem
* examples\GradientWidget.py: Demonstrate use of allowRemove
Co-authored-by: 2xB <2xB@users.noreply.github.com>
