13451: ALT key behavior with regard to main menu

[krasko78]

Resolves: #13451 - first part only (second one about the underlined letters needs a discussion)
Should resolve #19239 too although I am going to make an additional change for it (will leave a comment there in a minute)

A pretty simple and straight-forward fix. Works on Windows for me and since the user who has reported #13451 has stated Linux as their OS, I am not limiting this fix to any OS (for now).

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[krasko78]

Regarding the underlined letters: there is an open issue #15134 to not display the underlined letters in the menus if they are open with the mouse, so more opinions are needed here to decide which route we want to go: all always underlined or all underlined only when navigating with the Alt key. Personally I prefer to always see the underlined letters because when I want to open a menu with the keyboard, I look up the underlined letter first and then press ALT + that letter in a single quick gesture. I do not like having to first press ALT, then look up the underlined letter and press it as this feels slow and clumsy. Even if it is one of the modern trends - "modern" does not always mean better...