Added an ability to transform layers recursively with a Transform Tool

Now the transform tool supports transforming of a group of layers or
masks recursively in a single pass. You can use it for transforming a
set of layers grouped into a Group Layer or just transform a single
layer with all its masks (e.g. with transparency masks). This option
is enabled by default, but you can disable it by switching a small
button in the transform tool configuration docker. This button has a
icon of a small spider (the tool "crawls" through the layers :) ) [0].

It would be cool, if someone tested it a bit ;)

[0] - you can suggest any other icon for this, actually =)

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