Make it easier to create tiled plots

[Arturs Bekasovs]

This looks like a long-requested feature, but today Muon scientists in particular mentioned it once again.

It is currently possible to create plots with multiple layers, however this is nowhere near to trivial. You need to create a graph, then add layer to it and add curve to the new layer. Tiling 4 curves like that requires a decent amount of mouse clicks.

On the other hand, it's quite easy to plot multiple curves on the same layer - you just plot a list of spectra. Although it seems like a reasonable default, it makes sense to add an option to create new layers instead of adding curves to the same one.

Possible solution is to add another option to plotSpectrum Python func and a tick-box to "Plot spectra..." menu, where you select which spectra you want to plot (appearing when you drag&drop as well). Additionally, it might be a good idea to provide an alternative in a few other places, e.g. when you select certain spectra/bins in data view, you might have "Plot spectra/bin tiled" in addition to existing "Plot spectra/bin".

There should be some pre-cautions in terms of the number of tiles we can display at the same time.

[Arturs Bekasovs]

This is a duplicate of #8909.

[Stuart Campbell]

This ticket has been transferred to github issue ​9239