Add button in Project Manager to remove all missing projects

[qarmin]

This PR add a button to automatically delete missing projects.
Once the missing projects have been removed, a short summary appears in the console.
When there is no more empty projects, then button become disabled.

EDIT: Button now is not visible instead disabled, when there is not missing projects
EDIT2: Added break, when found first project, to not do empty iterations

Before clicking at button:

After clicking at button:

[Calinou]

I suggest hiding the button entirely if there are no missing projects listed, as it cannot appear while the project manager is open.

[akien-mga]

I don't see much use for this feature, it's quite rare to have missing projects unless you do a lot of engine testing like you do. And even then, you can do multi-select to remove several projects at once with Del or the "Delete" button. So I don't think it's worth adding 70+ lines of code for this.

[groud]

I think the feature is ok, that happens often to me to have to clean a lot of imported projects (happends for the demo ones). The thing is that it takes the same space as the other more important buttons, while it stay something quite unusual.

In a rework of the project manager, I would rather have this buried somewhere in a menu, but until then, I think it is ok to have it as a button.

[girng]

straggler projects get greyed out and you can also hold shift or ctrl and select multiple projects, then click remove

[KoBeWi]

I don't see much use for this feature, it's quite rare to have missing projects unless you do a lot of engine testing like you do.

There's also a possibility that you have to work on another PC and copy your editor settings, which happens to have saved project list. That's quite rare too, but happened to me enough times to create the original issue.

[fire]

@Chaosus 's ui is more clean.

It solves the problem with choosing the exact projects.

[akien-mga]

Thanks!