Allow adding an always present title to the timeline canvas

[rodarima]

We instrument several components by using multiple levels of detail, so the same execution can yield very different results depending on which level it is used at runtime. Therefore, it becomes extremely important that we glue together the information of which level is being used to the timeline.

We can use the title of the window to store this information, but this is not enought as some people use window managers which remove the title. Or it can become cropped in a screenshot.

I would like to add a label that is always inside the timeline canvas, so it becomes much less likely that it dissapears. It also should appear on the saved image of a timeline.

Here is an example of an screenshot of the timeline when the user has a WM that shows the title:

And here is another one with an WM without the border (or cropped screenshot):

As you can see, in both cases the information is still visible.

I recommend occupying the area just above the timeline grid, so we leave the top left corner empty (above the row labels).

We need to be able to generate that label from runtime, but a single .prv may have different labels per timeline. So probably, the only option is to add a new attribute in the .cfg files, so we can define one per timeline and then we generate the .cfg files at runtime as well.

Something like this?

window_name CPU: nOS-V subsystem of the RUNNING thread
window_title "This is a custom label that is controlled by the user. Level=3"

If window_title is not pressent, the current behavior continues as-is.