Show a "You should sync" message if necessary

[bitspittle]

A full Gradle sync is required for us to have up-to-date Kobweb metadata information. However, if a user just installed the plugin, they won't have that information and won't realize it. So we show them a notification to let them know.

The user shouldn't see this popup if...

• they don't depend on Kobweb anywhere throughout the project
• they already did an import through some other means (i.e. if you open a new Kobweb project from scratch, it kicks off a scan that way)
• they finished a scan in a previous IDE session

We also use this information to avoid caching Kobweb state / processing Kobweb functionality if the plugin is considered disabled for the current project.

[bitspittle]

OK, here's the same basic change with the PersistentStateComponent gone. I'm going to abandon the other PR.

We do still use PropertiesComponent for a small bit of persistence, but it's essentially a single extension property now (see kobwebPluginState) and not a full PersistentStateComponent class. Much simpler!

[bitspittle]

Comments responded to, thanks for the great review!