Add more shortcuts to DisassemblyContextMenu

[ITAYC0HEN]

Detailed description
This pull request adds three shortcuts for DisassemblyContextMenu

1. P to define a function
2. Shift+P to edit a function
3. U to undefine a function

in addition, this PR will also add an overload method for createFunctionAt to create a function without specifying a name. This will be used in another PR.

On a side note, I really don't like this whole get*Sequence() things. We better have a proper shortcuts structs, globally.

Test plan (required)

1. Open a binary in Cutter. Got to a function and press U to undefine it
2. Press P to redefine it
3. Press Shift+P to edit the newly defined function

[xarkes]

Lgtm.