Add Edit menu entry for wrapping subexpression in parens

[Helder Correia]

The functionality already exists from solved issue #501. Shortcut is Ctrl+P. The goal of this task is to spread its existence and make it easier for the user to get to know about it.

[Pol Welter]

Wouldn't that mean that the code for this should be migrated from the editor to the mainwindow?

EDIT: What I mean, right now, the shortcut works only when the editor has focus.

[Helder Correia]

See pull request #74. Thanks @polwel , one less thing to do when I go back from vacation :)