Program.cs: fix "Engine not prepared yet" error when player start a mission without first joining Springiee. (happen because Set engine only happen when joining battleroom)
Cleanup, UnitSyncRetryPrompt.cs & SpringsettingForm.cs don't need to set engine path
MissionBar & SinglePlayerBar: download & wait for default engine (if don't have it yet)

Utils.cs: made VerifySpringInstalled() check if Spring is really there.
Merge pull request #289 from xponen/zkl_fix_engineNotPreparedYet

ZKL: fix "Engine not prepared yet" error (fix #272 )