AddonManager: Changing the stop icon from green to red

[maxwxyz]

fixes #13563

Using a red square icon instead of a green one.
@chennes FYI

[maxwxyz]

@chennes alternatively there is also process-stop.svg in the same directory, maybe that is also a good fit (but it is also used for the close button in the addon manager).

[Reqrefusion]

Something more circular will be made for the Close button, which can be used for the purpose here. An octagonal icon seems more appropriate to stop than to close.

[chennes]

No strong preference here: whatever @FreeCAD/design-working-group wants to go with, I'll merge.

[MisterMakerNL]

Cross is close or cancel, square is stop.
We all know this from our video players. 😉
If you press this button does it saves the already loaded data? If so I'd see it as a stop button.
If you'd lose all already read data it's a cancel button.

[maxwxyz]

OK, then I'll leave the PR as is :)

[chennes]

If you press this button does it saves the already loaded data? If so I'd see it as a stop button.

It's a mix: the startup process is really six or seven independent processes. You will be cancelling the process that is running at the moment you click the button, but will be preserving the data obtained in any step that ran to completion prior to your click.