Invalid hotkey crash the whole app

[ZeroX-DG]

Current behavior

CodeMirror consider the last key in the hotkey sequence is the keyname and before that are modifier keys. For example:

Ctrl    +    Shift    +    V
   |           |           |
    \         /            |
     \       /             |
     Modifier             Key 

If the hot key is set to

Ctrl + Shift + V + B
  |      |     |   |
   \     |    /    |
     \   |   /     |
      Modifier    Key

which is invalid because V is not a modifier key. This problem will crash the whole app and cause this issue to happen: #3481

Expected behavior

Boostnote should validate hotkey before saving them.

Steps to reproduce

1. Open hotkey setting
2. Change a editor hotkey to Ctrl + Shift + V + B
3. Save the hotkey
4. Profit Crash

Environment

• Version : 0.14 but probably all version of Boostnote is affected
• OS Version and name : Linux, Windows, Mac