opt: revise QKeySequenceEdit usages

[shenlebantongying]

• set Maximum length of global shortcuts to 2
• set Default to something that can be set by user

Note that

On Qt's side, QKeySequence → Ctrl+C Ctrl+C are two press of the combination of Ctrl+C.

However, the existing global shortcut only sees the first combination and the 2nd combination's key

v    v      v
Ctrl+C Ctrl+C
       ^ this one is ignored by GD

There is no one complaining about this little flaw, probably because no one crazy enough to use different modifiers for continuous key presses 😅

[sonarcloud]

Quality Gate failed

Failed conditions
C Maintainability Rating on New Code (required ≥ A)

See analysis details on SonarCloud

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