Option to use only left or right modifier for key mappings

Currently when mapping keys for a VM in preferences, there are only generic shift/control/alt/cmd toggles that apply to both the left and right key. This is fine for US and I think UK keyboard layouts, however for some international layouts this poses a bit of a problem.

With the keyboard layout I use (Danish), you type AltGr/Right Alt + 2 to make an @, this also works with either right option + ' with Parallels's Mac layout it installs in Windows, or right option + 2 with the default one. However, now if I go into the settings and map cmd to option, and option to cmd, so the buttons on the keyboard have the same position as on a PC, both left and right cmd are mapped to left option, and left/right option are mapped to left command, making pressing key combinations that require AltGr impossible. This can likely also be a problem if someone tries to create certain keymaps to enter characters that require a combination of modifier keys.

If you try to map left alt to AltGr, to make entering symbols work like in macOS, there's additionally a problem because in Windows, AltGr+Tab acts differently from Left Alt + Tab, trying to fix this by creating an additional mapping of Option+Tab to Alt+Tab does not work, as Windows does not seem to pick up holding the Left Alt key after pressing tab, making it impossible to switch between more than the two most recently used windows.

I'd suggest adding options in both To and From in the key mapping dialog to pick whether or want the left or right key for a modifier to resolve this issue. Could perhaps be under some sort of advanced option to avoid confusing people who do not use keyboard layouts where this matters.