Conflict solver: add the ability to manually select which conflict solver to use

I like the possibility to set different solvers for different file extensions, but sometimes I would like to manually select the solver.

I'm working with PLC code files and, for some of them, a specific graphical solver is provided. Sometimes (I have to say often), that solver fails, but on the low-level, those files are XML-like files, so a manual intervention is possible.

It would be great if you could provide a drop-down menu, with all solvers feasible for that file type and let the user choose.
Of course, the "Conflict Solver" command should behave as now, launching the default one.