Better cross-platform support

[calmofthestorm]

We should add some additional classes to the core to make working across platforms easier. You can currently do it with ProxyPlatformContext, and that may be necessary for more complex cases, but simple things like different keystrokes should be much easier.

• ProxyPlatformAction -- performs a different action based on whether currently active is local or a server, and if a server based on what platform (and possibly WM?) it reports.
• ProxyPlatformKey/Mouse -- sugar for ProxyPlatformAction with Key objects (IE, enter different keystrokes based on platform).
• Some kind of custom RPC context that makes it easier to work with plugins maybe? Like ProxyRPCContext("get_vim_mode") would call the get_vim_mode RPC (added by a plugn presumably) and match based on the value. Maybe we don't need this and should let plugin authors do it themselves, I welcome opinions here.

Anything else in the same vein?

[calmofthestorm]

Related to the conversation previously at #65, take a look at ContextAction for the time being. It's not as good as what's proposed above but it's less painful than custom rules.