Add clock sync check before local node startup

[avive]

On app startup, before starting a local node, compare local system time with time from a public time server.
If time is out of sync or can't reach a reliable time server then show the standard modal alert box with the following content:

Title: Clock out of sync
Body: Your computer clock is out of sync with network time. Please synchronize using the data and time settings.
[Quit] [Retry]

• Retry should attempt to get network time and check again if local clock is out of sync. If synced then proceed to start local node.
• Quit should quit Smapp.

[avive]

Addressed in 0.2 by properly handling node panics on startup. Might still be a nice 0.3 enhancement.