Separate the selection tool logic from the figure splitting tool

[stelfrich]

@mutterer via e-mail:

I thought again about the current code design in FigureJ, and I think a first step that should be taken for a redesign, would be to separate the selection tool logic from the figure splitting tool. Thus, it would be easy to invoke the selection tool on any opened image. It could even be useful for itself it made compatible with the builtin crop command. In FigureJ, however, it could be leveraged in the current 'crop+scale+fit' way.