tools: Add more toolbox convenience

I have two scripts - meson-build and gs-run - that I use extensively during development.

Those may be useful to others as well, so I cleaned them up and propose to include them in tools/toolbox.

Input welcome (useful additions, removal of pointless options, outright rejection, ...).

If we decide on include the scripts (in some form), I'll try to find some time to add a small README.