yuzu qt: Create shortcuts on Linux

This creates a Desktop Entry file and a PNG icon for the entry when the
user right-clicks a game and selects "Create Shortcut -> Create
{Application,Desktop} Shortcut". This uses the current executable's path
to create the shortcut.

yuzu qt: Add more error checking and OS gating for shortcuts

main: Remove FreeBSD gating for shortcuts

I'm not going to test FreeBSD, so I don't know if they follow
Freedesktop.org or not. I just have to let someone else verify that it
works there and let them enable it.

main: Move shortcut function to its own function

This function should really be in a common library, at least among
frontends.

main: Remove image manip references

main: Fix difference in MinGW and native GCC versions

main: Fix negation in creat shortcut

Addresses review comment

Co-authored-by: Jan Beich <jbeich@FreeBSD.org>

main: Re-enable freedesktop shorcuts for FreeBSD