MainWindow: move shortcuts from .cpp to .ui file + change Clone Entry shortcut

[c4rlo]

Where possible, move shortcuts from being defined in MainWindow.cpp to being defined in MainWindow.ui. Previously, there was a seemingly random mix of both practices.

The only shortcuts defined in the .cpp file are now ones that can't be defined in the .ui file, because they are in some way conditional.

This also reduces the number of compiler warnings of the kind

warning: arithmetic between different enumeration types ‘Qt::Modifier’ and ‘Qt::Key’ is deprecated [-Wdeprecated-enum-enum-conversion]

with recent GCC versions.

And some minor code cleanups in MainWindow.cpp.

Also, change the shortcut for the Clone Entry shortcut from Ctrl+K to Ctrl+D (see discussion below).

Testing strategy

Type of change

• ✅ Refactor (significant modification to existing code)

[droidmonkey]

You need to mark all the shortcuts in .ui as not translatable

[c4rlo]

You need to mark all the shortcuts in .ui as not translatable

Ok, done.