UX for target selection in cmake build steps

This is a follow-up from QTCREATORBUG-24145 (see comments in that issue).

Not sure if "Bug" is the right type, but IMHO the UX for the common case was better in 4.12.

My workflow when switching the build targets is that in 99% of cases I want to switch between "current executable" and "all". IMHO this should be a very common use case, if not the most common one.

This used to be very simple, since apparently you could only ever select one target up to 4.12, so it was easy to toggle (I though it was special behaviour of the special targets, since it always used to be checkboxes, not radio buttons).

It's great you can select multiple targets now, but it seems to me a bit more of an exotic use case, so I'm not sure it's worth making (what I assume is) the common use case more time consuming.

My suggestions:

• Treat special target "all" specially, such that selecting another target unselects "all" and such that selecting "all" unselects other targets
• "all" should be always sorted at the top, just like "current executable". "Current executable" should remain the first one.
• Moreover, selecting no targets at all also makes no sense since it looks like that results in an invalid cmake command invocation. So a convenient behaviour could be automatically selecting "all" when unselecting the last target, and maybe switching to "current executable" when unselecting "all" (assuming this is the most common choice). This allows for easy toggling between all and any other target by just clicking that target.

It might not seem like much, but as someone in the comments to the 4.13 release announcements put it (https://www.qt.io/blog/qt-creator-4.13-released):

Not to mention that, it the previous versions, once you select the 'Current executable' as a build target, the 'all' was removed. Now, I have to scroll through the list and locate the all and unchecked it manually. Tiny things that drive crazy.