Move handling of -tmp parameters from script to Main

[johann-petrak]

Currently the parameter -tmp is essentially a work-around for the quirky way of how config and session files are handled and also for how the default directory shown in the file picker depends (or not) on the current directory when GATE is started (always starting from the user home is annoying).

This should get handled properly from directly inside GATE, but for that the way of how config and session file settings depend on each other needs to get untangled.