Draft: Issue #421: Add option to create selection from layers

This draft patch attempts to implement the option requested in #421. The current Select All feature selects the entire image. If you run it on a layer that's smaller than the full image, the selection appears to "grow" when you switch to a larger layer.

This patch adds a "Select Layers" option below this that only selects the regions around the current "Active" layer(s). When you switch layers, the selection stays the same size. "Select All" was also renamed "Select Canvas" as a potential way to distinguish the two actions (the mnemonic remains the same).

Note: In the bug report, there was discussion of whether this should be a separate option or whether this behavior should replace the existing "Select All" behavior. I have included it as a separate option in this draft, both for comparison and because replacing the existing behavior is a lot easier than doing the opposite. 😄