This patch implements the generic slider created in `ed_draw.c` (D9314) in `graph_slider_ops.c` for the `GRAPH_OT_decimate` operator (activated 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
{F10376115}
(Colors subject to change with D11878)