flint: Readability of image marker at BM18

Feedback from BM18.

Here is a screenshot 1 to 1 of the screens at BM18.

The image takes a lot of space.

And marker are difficult to read.

To improve that there is different approches.

1. For now the color of the ROI is the same as the color of the text color
   • Ask for a text-color API in silx (so what we could choose a text color with a better contrast, which could be black)
2. Maybe the pink color (for the marker and the text) is not the best one
   • As discussed with Paul, maybe a "green" would be meter in this case
   • silx provides a "marker" color per colormap, i don't think it's the color which is used right now
3. In such screen the text is also anyway very small
   • As discussed with Thomas, we could ask for a global text size configuration in silx
4. Finally, if you care about the image size (indicator displayed at the bottom-right of the screen) we could place it at a better location
   • We could imagine a Qt overlay (HUD) on top of the image with some information
   • As it would be a Qt widget, there is dedicated API to request
   • Such overlay could be half-transparent and could disappear when it is hovered by the mouse cursor
5. The 0,0 and image size are markers with only text, the API already allow to change the text color
   • So we can choose to display it in black already

FYI @tvincent @meyer