Fix crash on UNDO/REDO shortcut press when in "title screen"
### Problem description
ImHex crashes when (by default CTRL + Z/Y) undo/redo is pressed when on
"title"/"starting" screen (no file open, tested on Windows release build
and Linux [WSL] from-source build).
This is due to the shortcut's callback being called even if the
`provider` is `nullptr`. (see `createEditMenu` function).
Theoretically, this is prevented by the `enabledCallback` function
passsed to `addMenuItem`. In this case, though,
`addMenuItem` correctly propagates `enabledCallback` to menu item
creation but does not pass `enabledCallback` to
shortcut creation. Thus, when handling shortcuts, `enabledCallback` is
not used at all and the shortcut's callback
can be called in contradiction with its preconditions. (specified by
`enabledCallback`)
### Implementation description
The implementation wraps the callback in a check that decides whether
the shortcut is enabled or not.
(see changed files)
```c++
auto callbackIfEnabled = [enabledCallback, function]{ if (enabledCallback()) { function(); } };
```
This function is then passed along instead of the `function` (shortcut's
callback).
Alternatively, we can check for `nullptr` in the callback directly. This
would require modification of `createEditMenu`'s contents.
(I did not choose this implementation because I do not think it
addresses the root of the issue).
### Screenshots
None
### Additional things
I'm not sure how big of a deal it is but I am unsure whether I can
capture (`[enabledCallback, function]`) by reference or not.
d9a7f40eb4