Investigate method of saving java commandline from gradle to speed up launchClient

[DaveyBiggers]

A recent improvement to the launchClient script (not yet in master) dumps the command-line into a new file - runMC.sh - which will be used to avoid having to build Minecraft the next time launchClient is run.
It should make startup times significantly faster, and avoid some network access issues.

On linux systems, however, the generated script doesn't have its executable bit set, so launchClient.sh fails to run it.

[DaveyBiggers]

While we're at it, on Windows the command line doesn't run, because the path to the java executable isn't quoted. (eg the space in "program files" causes an error.)

[DaveyBiggers]

I've also seen problems similar to #542 - might be that something is going stale as a result of dev work on the mod, in which case this approach might still be viable for users, who won't be changing the java code. Or maybe this trick just isn't reliable enough. For now I'll keep the generation of runMC, but remove the code from launchClient which uses it. Then users can choose which script to run, but launchClient should always work.