Interface for adding/editing "search_path" setting

[abathur]

The search_path key is used by the "Add project by file" and "Upgrade & add workspace" commands as the directory in which they begin searching for sublime project/workspace files.

• Add an interface for managing this key (API support is already available)
• A sane default if one is easy to set (I would prefer the commands to be disabled until the user acts if there is no sane default).
• Better messaging/communication of why the commands are disabled or what the user should do about it? (Disabling is a little opaque as is; if you ignore the readme and don't look at GH issues, I doubt you'd figure out why they aren't lit up.)