Make the tool box fixed on the left with a single column.

[jpobst]

By fixing the Toolbox on the left and restricting it to a single column, we can switch to using GTK's built-in Toolbar instead of the custom wrapping container we currently use. This has the benefit of properly handling overflow items if there is not enough space to show all items, which our current toolbox doesn't do.

This also sorts the tools to be better grouped for a single column, and to place to "more important" tools higher so they do not go to overflow. The decision of what is "more important" is completely subjective, so it can be changed based on other opinions.

[cameronwhite]

This looks good, thank you!
I think the ordering of the tools is reasonable - on my machine, with the default window size only the Clone Stamp and Recolor tools were in the overflow list