**Slider upgrades**
This patch adds a generic slider that can be used for `GRAPH_OT_decimate` and for the pose sliders in `pose_slide.c`.
**
Usage**
* an operator that wants to use this slider needs to save a pointer to `tSlider`. An initialized `*tSlider` is returned from `ED_slider_init` for that
* call `ED_slider_update` to update the percentage value and the drawing
* use `ED_slider_status_string` to get a string with help info that can be printed to the status bar
* use `ED_slider_exit` to free the memory and remove the drawing callback