Added undo and redo in gear activity

[hharshas]

#838 @llaske added the feature of undo and redo around deletion and addition of gears/chains, which will now enable you to track its location as well.

[llaske]

Nice.

I've got two issues:

• [x] Icons Undo/Redo don't appear
• [ ] Gears are not always recreated at the same level (see the orange gear on the top of the magenta one)

[llaske]

Icons are now visible. BTW it will be nice to add a vertical bar before the undo icon.

The level issue is still there. If it's really complex to fix, we could debate about it.

When reopening an activity and trying to undo, the whole screen become blank.