Turtle Art: Put '-' icon on top icon for collapsible block

[Mokurai]

When creating Turtle Art stacks too long for the visible workspace, one can frame code segments with two unnamed blocks from the Extras palette, allowing the programmer or user to collapse code to a single block. The lower block has the '-' icon for collapsing the segment. However, this means that the programmer or reader of the program has to scroll down frequently just to collapse blocks. Putting a similar '-' icon on the upper block will remove this necessity, saving significant time, particularly when debugging several long stacks.

[walter]

Nice suggestion.

See  http://git.sugarlabs.org/turtleart/mainline/commit/68532a1ae4097a5480ee46e1ea240244549ac60b

Will be in v114