Use threading to download assets to speedup startup

[rondlh]

Use threading to download assets to speedup startup

Reference: #100

[rondlh]

This PR improves startup speed for me significantly. I'm not sure if this will cause issues for new user.
I noticed that if I change any assets while CaDoodle is running that this is detected and changes are reflected immediately.

[madhephaestus]

the reason this is done as a blocking operation is because the resources in those repos are necessary for further processing. they need to be fully cloned (and when it is the first run of a version or is source it updates the repos.

While this would speed startup, it needs to be blocking.

[rondlh]

the reason this is done as a blocking operation is because the resources in those repos are necessary for further processing. they need to be fully cloned (and when it is the first run of a version or is source it updates the repos.

While this would speed startup, it needs to be blocking.

OK, I guessed so already, but do they need to be blocking every time?