Simplify auto check for update

[hql287]

This is an improvement on how Manta should handle checking for updates.

Expected Behavior

Automatic

• The app will always check for updates on startup,
• If there's no update, the user doesn't need to know about it.
• If there's a new update, let the user know about it and ask what to do next - displaying a dialogue box with options (Ignore or Upgrade)

Manual
If the user explicitly checks for updates:

• If there's no update, DO let the user know by displaying a dialogue box.
• If there's new update, DO let the user know and ask what to do next by displaying a dialogue box with options (Ignore or Upgrade).

Goal

• This means the user doesn't even need to think about updating the app unless they want to.
• Simpler UI as settings like check for update frequency can be safely removed.

Current Behavior

Manta uses check-for-update-frequency setting value (daily or weekly) to determine whether it should check for update, which is not necessary.

Possible Solution

• Refactor (simplify) check for update helper
• Remove Check for updates Frequency option in Settings page

Your Environment

• Version used: v.1.1.3
• Operating System and version: macOS

[hql287]

Fixed with #262