SCUMM: Alt-F5 no longer work to get original save/load dialog

[criezy]

In recent daily builds using Alt+F5 to get the original save/load dialog in SCUMM games no longer work. It was still working in the 2.1.2 release. With recent builds it ignores the modifier keys and displays the GMM instead.

Git bisect points to the following commit as being the culprit (and it indeed seems relevant).

b4083476d5a8645e781052e68918a32a2e821dbf is the first bad commit
commit b4083476d5a8645e781052e68918a32a2e821dbf
Author: Matthew Jimenez <matthew.jimenez@outlook.com>
Date:   Tue May 26 22:00:49 2020 -0500

    KEYMAPPER: Allow mapped actions without modifier keys when no mapped actions exist for current modifier keys

I tested with Indiana Jones 3on macOS, but I suspect this is not specific to this game and this platform.

[bgK]

I've been thinking how to fix this issue while keeping the desired behavior for modifier keys mapped to actions. So far, the two best options I've came up with are:

• When an action is mapped to a modifier key for a given keymap then the modifier key cannot be used as a modifier for other actions in the same keymap.

I fear this would introduce usability issues unless there is a good UI to help understand what is possible and what is not when binding keys.

• When an action is mapped to a modifier key, the key mapper should lookup the actions for both the modifier + the key and the key (by order of priority) (only for the keymap with the action mapped to the modifier).

I'll take some time to think about the options before implementing a fix.

[ZvikaZ]

As @criezy noted at https://bugs.scummvm.org/ticket/12410 , these two issues seem related.
When addressing this, please try to take #12410 into account.