Create basic structure for E4 Window Commands Plugin.
Close #221: implement window commands for Eclipse 4 by @igrekster.

Ported code to commands in separate plugin, implemented provider and
keybinding.
Make getCommands() final to avoid confusion by implementers.

UnionStateProvider makes a union of the member variables.
Implementations shouldn't override getCommands() because only the member
variable counts.
Add .gitignore files.