Improved layer selection

[dmstraker 399]

At the moment you can select layers by clicking on them or shift-click to select a set, which is just fine.

It would be great to adopt additional common selection paradigms, such as:

• Shift-down to incrementally select.
• Shift-up to incrementally deselect.
• Ctrl-A to select all.

Ok, so some of these are already defaulted elsewhere, but it could still be done with key combinations (and I could reassign keys to the functions I use more often).

Cheers!