Visually indicate to users that they should click Refresh

[HebaruSan]

Background

Since its creation (see #2177), ConsoleUI has shown the time since the last repo refresh in its bottom right corner:

• 
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Problem

Lately, that text indicates the last time a mod was installed or uninstalled, not the last repo refresh.

Cause

As of #3904, the available modules are no longer stored in CKAN/registry.json, which is no longer written after a repo update, but this is the file that ConsoleUI checks for the last update timestamp.

Motivation

@JonnyOThan reports (see #4130 (comment)):

I just had someone tell me that they couldn't find tweakscale rescaled on ckan because they hadn't refreshed since it was released.

GUI users apparently need a cue/reminder to update their metadata.

Changes

• Now RepositoryDataManager provides:
  • A LastUpdate function that returns the time since the last update
  • TimeTillStale and TimeTillVeryStale properties indicating when metadata should be considered a little stale (3 days) and when it should be considered very stale (14 days)
• ConsoleUI now:
  • Uses LastUpdate to get the last refresh timestamp instead of looking at CKAN/registry.json
  • Uses those intervals instead of its previous values of 1 and 7 days
• When a GUI user's metadata is stale, a yellow notification-style dot (created by @JadeOfMaar) appears on the Refresh button, and its tooltip is set to a message suggesting to click Refresh
  
  • The same happens for very-stale but with a red dot
  • After refreshing, the dot goes away and the tooltip reverts to normal

Fixes #4130.