[FEAT] Have the region-select tool also be selectable from the aperture photometry plugin

[eteq]

Jdaviz component

Imviz

What is the problem this feature will solve?

When using the aperture photometry plugin, one has to know to click ouside of the plugin to create an aperture/subset. That's a bit disruptive of the mental workflow because I would have expected to click somewhere in the plugin.

Describe the desired outcome

Anything in the plugin that would allow this would work. A few options include:

1. Have the subset button from the toolbar appear somewhere in the apphot plugin. This would just be a convenient duplicate that would otherwise be identical in functionality to the button (and linked with it, presumably)
2. Have a special option for when no subset exists that's something like "create new..." which then programatically goes and "clicks" the subset button in whatever mode it's currently in.
3. Have some text that says "not sure how to create an aperture? click the icon that looks like to create a subset"

Additional context

🐱

[pllim]

(3) Might be the most feasible because the other ones will duplicate existing functionality in Subset Tools.

[kecnry]

The solution here could be combined with #2546 - if there are no subsets, show an alert dialog along with a button to activate the tool. The slight problem with that is there could be multiple viewers, so do we activate the tool in all viewers, just the default, provide a button for each viewer, etc.