Ability to change the favicon.

[fath11]

I think this will be a great addition to the System Tools extension since some creators in the international version want custom favicon for their games.

Unfortunately, I can't get the extension that is in the developer branch to load in gandi so I am just making this issue so anyone who CAN get it to load will add this feature.

[fath11]

@yuen619

[fath11]

in a situation where no one bothers to do this, I want to know if I have permission to port that darn extension to the normal extension format so it can go into the normal extensions branch(main branch) and no one has to overcomplicate anything

pros in doing the above: the extension will be compatible with every other scratch mods

[MoreBugOfDog]

这个拓展我正在做
请静候我的PR！
当前积木：

[MoreBugOfDog]

PR # 253