Dynamically resize wb splash screen image based on screen resolution

Settings docs update

Fix Colorfills Scaling Differently if Done From Figure Options or the Context Menu

This should resolve some of the Huge splash screens we get with the workbench sometimes.

It also means we only have to maintain one splash screen image and not two resolutions of the same thing.

This commit converts the PSIBackgroundSubtraction algorithm to a data processing algorithm. This increases the clarity and transparency of the workspace history.

In order to do this, the operator overload which removed the background has been replaced with a CreateWorkspace call, which generates a workspace based on the backgrounds obtained, followed by a call to the child algorithm "Minus".

un-capitalise and remove unnecessary plural

Co-authored-by: Conor Finn <47181718+ConorMFinn@users.noreply.github.com>

Co-authored-by: Nick Draper <nick.draper@stfc.ac.uk>

Co-authored-by: Nick Draper <nick.draper@stfc.ac.uk>

The Poco headers, and the function referencing them appear to be unused.

Updating unit tests based on changes

Move include from header to cpp file

This was noticed after #28655 was merged

A waterfall plot with fills would leave the fills behind when the
workspace was deleted. Now does a check that the plot is a waterfall and
has fills before updating the fills on the plot.

If anything was changed on the colorbar, it would not be removed when
the workspace was, resulting in a broken, empty colorfill plot. The
check is now that an axis has no lines or images, rather than checking
it's type.