Added ShortcutOverride handler for Backspaces, fixing plot delete

[ChrisYx511]

Description

Added a QEvent.ShortcutOverride catch to the utils.py event filter such that as when clicked into a pyqtgraph plot item, the backspace key throws a ShortcutOverride rather than a standard KeyEvent. This allows deleting plot items and any other items depending on pyqtgraph properly.

Changed EventTracker eventfilter() function to also catch and handle QEvent.ShortcutOverride types if and only if the key pressed is also Qt.Backspace_Key in order to fix the above issue while not causing any unintended side-effects or breaking any default keyboard shortcuts.

This PR closes #306 .

Developer Testing

Test all existing keyboard shortcuts with this change and make sure they still work.

• Pressing backspace in a graph works in all conditions, wherever i click on the plot item (outer box, inner plot, edges)
• Press all other keys and make sure they still do their intended functions.

Reviewer Testing

Here's what you should do to quickly validate my changes:

• press some keys
• make sure the plot item deletes no matter where they're clicked or focused

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This change is 

[zangjiucheng]

Reviewed 1 of 1 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @BigLiban, @Darth-Raazi, @Lucifersan, @patrick-gu, and @saads312)