UI improvements: New account wizard

[camilasan]

Add account option

• On the system tray menu, change text from "Add account" to "Add Nextcloud account"

First step of the wizard (login or sign up)

• Change wizard window title to "Add Nextcloud account"
• Align titles: "Connect to Nextcloud" and "Setup Nextcloud server"
• Change background to Nextcloud blue + white font (see Android client)
• Stop automatic carousel animation: allow the user to navigate through it only
• Make login button the primary action
• Change text "Sign up with Provider" to "Create account with Provider"
  • If user clicked on "Sign up with Provider"... on the next step: make webview window smaller and remove white border
• Add text "or" between buttons

Second step of the wizard (server url)

• Remove button to sign up with providers from this step
• Remove 🔒 icon displayed by default: it should only show up after if user enters an url
• Change hint text color to grey (the texts that explains how to fill up the url)
• Change text "Next" in button to something like "Pick syncing folders" (or what describes "next" step)

Third step (syncing folders)

• Replace the Nextcloud (application icon) logo with the user avatar
• Replace text "Server" by username and server name

New fourth step

• Add a 4th step where it says new account is connect and syncing
• After it should show the main dialog

cc @jancborchardt

[elsiehupp]

Hi there! I added a pull request that fixes the issue of not being able to quit from the first-launch wizard, and I figured I should mention it here just so we can avoid any potential conflicts.