Adopt Multi Root Workspace APIs

[JamesGiller]

Since the latest stable release included support for multi root workspaces, I guess it is important for this extension to follow the guidelines for adapting to the new API.

In particular, I have found references to the now deprecated workspace.rootPath:
master branch

src/catkin.ts
27:  const cwd = typeof uri !== "undefined" ? uri.fsPath : vscode.workspace.rootPath;

src/build.ts
18:  pfs.exists(vscode.workspace.rootPath + "/.vscode/c_cpp_properties.json").then(exists => {
30:  const filename = vscode.workspace.rootPath + "/.vscode/c_cpp_properties.json";

src/extension.ts
39:  baseDir = await utils.findCatkinWorkspace(vscode.workspace.rootPath);

4-catkin-tools-support branch

src/extension.ts
44:  baseDir = vscode.workspace.rootPath;

src/build.ts
18:  pfs.exists(vscode.workspace.rootPath + "/.vscode/c_cpp_properties.json").then(exists => {
30:  const filename = vscode.workspace.rootPath + "/.vscode/c_cpp_properties.json";

Also, for the commands like creating a package/running a build, we should probably ask the user which workspace folder they want to add the package to/run the build in.

Any other ideas, in accordance to the changes as listed in the linked wiki?
I will read further into it.

[ajshort]

I don't think it would be common have more than one ROS workspace open at a time. I think a good solution is to scan all vscode workspace folders for all ROS workspaces, and if there's more than one unique one either:

1. Show an error.
2. Show a picker for the user to choose one.