Fix codepage and language file switching issues

[maron2000]

What issue(s) does this PR address?

Some bugs in changing codepages were fixed to address the following two issues.
Keyboard layout is now unchanged while switching codepages.
Codepage is changed appropriately when loading language files using CONFIG command so that menu bar is not glitched.

Tested on VS and MinGW builds on Windows 10 Pro 22H2.

Fixes #5207
Fixes #5208

Keyboard layout unchanged when switching codepages.

No menubar glitches when loading language files using CONFIG command. (normal and TTF output)

[Torinde]

@maron2000, maybe for a separate PR, can you please add a keyboard combination to switch codepages on-the-fly? The usecase is that when using a non-Latin script codepage (e.g. Cyrillic) the box drawing characters of most installers and programs are missing/rearranged (because they're hardcoded for 437).

It would be great if KEYB or DOSbox-X itself supports a key-combination to change between 437 and whatever codepage is active - for example 3021 moves around the box drawing characters and back in the day we used TSR programs allowing LeftShift + RightShift to change between the codepages on-the-fly (when you need to see something like a game installer menu, which is written in 437, but soon after you'll change back to 3021 in order to read/write in Cyrillic).

• when codepage is 437 pressing LeftShift + RightShift changes to the 'alternative' one (e.g. 3021 or whatever was set by CHCP or KEYB)
• when codepage is <>437 pressing LeftShift + RightShift changes to 437

To illustrate why LeftShift+RightShift combo is needed for user to switch to/from 437 - see how EDIT looks with 3021:

While how it should look:

[maron2000]

@Torinde maybe your request will be a feature request rather than putting it here.
I don't know now whether to add such feature is trivial.