This patch implements the generic slider created in ed_draw.c (D9314) in graph_slider_ops.c for the GRAPH_OT_decimate operator.
Activate the operator with the "Decimate (Ratio)" button in th "Key" menu.
This means:
- Hand over percentage calculation to tSlider
- change the way the status message is created and draw it in the workspace footer
(Colors subject to change with D11878)
