Use welcome view when the extension can't show TreeView

Problem to solve

There are several scenarios when the extension can't do any useful work:

• New VS Code window (no workspace is open)
• None of the open workspaces contains a configured GitLab project
• The personal access token hasn't been set up
• There workspace folder doesn't have initialized git repository #321 (closed)

For example, attempt to use the extension when there is no open project now results in an error message:

Proposal

Utilise the Welcome view available in for VS Code extensions:

Scenario 1: No token

Hey. Welcome to the GitLab Workflow extension. It seems that you have not yet set up your access token.
If you use GitLab.com, click [here](https://gitlab.com/profile/personal_access_tokens). If you have a self-managed GitLab, then: 
1. Go to 'Settings'
2. Click 'Access Tokens' on the left navigation menu
3. Click on 'Add a personal access token'
Now you can set your token by clicking on the following button
[Set Personal Access Token](command:gl.setToken)
To learn more, please [read our docs](https://gitlab.com/gitlab-org/gitlab-vscode-extension#setup).

• mention that the token needs read_user and api scopes to fix #230 (closed)

Once this scenario is implemented, we should not show the "add token" info message during every extension startup. It was bothering users: #279 (closed)

Scenario 2: No GitLab project

It seems like your current workspace doesn't contain a GitLab project (or the project has been moved).

Scenario 3: No open project

In order to use the GitLab Worfklow extension, please open a folder containing a GitLab project.
[Open GitLab project folder](command:vscode.open)

Further details

There is a small POC implemented in welcome-poc branch.

Examples

extension	screenshot
source control (no open workspace)
source control (no git repository)
debugger