fix: tool usage with tablet eraser

[letsfindaway]

This PR tries to fix #437.

• use correct tool after switching to/from tablet eraser
• re-evaluate currentTool after changing it by the tablet eraser

However I need some testers, as I do not have a tablet with an eraser function. I will add this PR as a patch to my experimental builds, so that experimental packages are available later this day.

@bruening-bw : there will be packages for several versions of Debian available. So it would be helpful if you could test whether this fixes the issue for you.

[bruening-bw]

The eraser works as it should. When I use the eraser on the back end of the pen, it works immediately. And if I use the pen again with the tip, it switches back to the drawing tool and is pressure sensitive from the first stroke. This works with the pen and the marker and the straight line tool.

If I have selected the laser pointer and then use the eraser, it does not switch back to the laser pointer afterwards, but to the last drawing tool used before. The same applies to all other tools that are not used for drawing, such as scissors, the selection tool, text, zoom, hand, etc. But that's not a problem. No need to bother with it.

I have also tried the eraser in desktop mode. It works great when erasing on the desktop.

I didn't notice any unwanted side effects during my tests.

[letsfindaway]

If I have selected the laser pointer and then use the eraser, it does not switch back to the laser pointer afterwards, but to the last drawing tool used before. The same applies to all other tools that are not used for drawing, such as scissors, the selection tool, text, zoom, hand, etc. But that's not a problem. No need to bother with it.

This is something which could be changed, too. But I would classify it as a "behavior" and not as a "bug" and would like to have some feedback from @kaamui before changing the behavior.

I didn't notice any unwanted side effects during my tests.

Thanks for testing. And thanks for bringing up this old topic again - I think it was just forgotten.

So I hope it will be fixed in the next release of OpenBoard.