Show correct context menu when using Menu key

[PremadeS]

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Detailed description

Opening context menu using the "Menu" key from the keyboard opens a generic context menu as if the user clicked outside of any instructions, both in disassembly and graph widgets. This PR fixes that specific case

Test plan (required)

Since not a lot of systems come with the "Menu" key, one can remap a key to behave as "Menu" key to test this PR

Wasn't able to find a working solution to get the "Menu/Application" key working on macOS both with and without remapping
See context-menu-right-click-keyboard-shortcut-in-mac-os-x

Steps:

• Open cutter with or without binary
• Go to Disassembly widget
• Press the "Menu" key (or the remapped key)
• Test the actions within the context menu to make sure they work for the currently selected instruction
• Repeat for Graph widget, also test when no graphs are available

Note that context menu opened through keyboard appears in the middle of the widget that currently has focus, (that's handled by Qt)

Closing issues

closes #2107

[Rot127]

Works for me