Usable available memory checking

[ghost]

Currently available memory checking is done when adding tasks, which prevents adding succeeding tasks when current task is running. A better way is checking available memory on tasks begin.

[AkarinVS]

Branch custom/no-free-memory-check patched the source to remove the check.

Look for binary releases here:
https://github.com/AkarinVS/OKEGui/actions?query=branch%3Acustom%2Fno-free-memory-check