Allow for complete override of GUI-set QEMU arguments

[CONIGUERO]

There are times in which it's necessary to add parameters to the "core" qemu arguments apart from the "machine" one, for example with -device vga,edid=on and many others.

Many of these optional params are not available in the settings GUI, and since the core arguments are locked down to the options specified in the VM settings it's impossible to ever run a vm with them.

I propose the following solution:

• Let the user customize all the QEMU arguments through the QEMU tab
• Add up/down arrows on all the arguments so that the user can easily customize the order as if they were writing it on the command line
• When an argument that doesn't have a GUI equivalent is detected, add a "Custom" label to the corresponding UI dropdown menu/box in the settings panel to indicate that the argument/setting is currently mapped to a custom value that doesn't have a GUI equivalent yet.
• Allow the user to override their custom arguments by simply selecting a known configuration parameter from the GUI again.