Better feedback for changes in the new settings widget

[jhaigh0]

Description of work

It was requested that the 'Apply' button on the new settings widget is greyed out when the user has made no changes to the settings, and enabled when there is a pending change.

The other thing I discovered in this PR is that the test files that were added in the original PR (#38373) were not added to cmake so haven't been running. I thought the search for missing pytest files pre-commit job was supposed to flag this. They've been added in this PR (along with new tests).

Summary of work

Design-wise I thought it best that the model of the settings widget tabs didn't know about the presenter. Instead, the presenter emits a signal when it adds a change in the model which is picked up by the parent presenter. For this I added a QObject base class of the presenters.

There is no associated issue.

Further detail of work

To test:

Play around in the settings widget, change settings and change them back again. 'Apply' button should enable and disable as you expect.

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[cailafinn]

I thought the search for missing pytest files pre-commit job was supposed to flag this.

To answer this, the precommit hook relies on searching for a call to unittest.main(), which it seems like these files are missing. Normally CMake catches this, but if they're the only test files in the directory, I think CMake overlooks them. Might be worth investigating this further and modifying the hook to account for it.