* Fix and add menu accelerators: menus common to all workbenches
I use menu accelerators fairly often, so I find it very frustrating when
they are missing, or worse, they don't work due to the same letter being
assigned to several commands.
This patch adds accelerators to lots of menu entries missing them and
fixes broken accelerators.
Wherever possible, standard accelerator keys are used:
https://doc.qt.io/qt-5/accelerators.html
This commit covers accelerator fixes that are common to all workbenches.
Accelerator fixes for specific workbenches will be done in separate
commits.
* Add missing accelerators: Spreadsheet workbench
