Changed the workspace proxy to reuse its cached algorithm instance

The class now only creates a fresh instance if:
 1. the current instance is empty (so on first use)
 1. It is about to execute, synchronously or asnchronously
 1. Execution has completed (here it just wipes the old instance, doesn't create a new one.)

This reduces the creation of algorithm instances to (1+1 nExecutions) per algorithm dialog
from (1 + nProperties + nExecutions).

This has merged in the changes from #12835 (keep open for alg dialog boxes)
as it is important for testing.

re #9480