Selection doest not obey SHIFT + arrow keys in Song Editor

#### Steps to reproduce
- Open a project. Select any part of the project. Try to use shift + right arrow key.
#### Expected behavior
Same behaviour as you can find in the piano roll

#### Actual behavior
Nothing happens.

#### Images

This is only possible using the mouse. However, the behaviour should be consistent, as in the piano roll.

#### Affected LMMS versions
Version 1.3.0-alpha.1.104+g4589004e6 (Linux/x86_64, Qt 5.15.2, GCC 10.2.0).

Operating System: Manjaro Linux
KDE Plasma Version: 5.21.4
KDE Frameworks Version: 5.81.0
Qt Version: 5.15.2
Kernel Version: 5.10.34-1-MANJARO
OS Type: 64-bit
Graphics Platform: X11
Processors: 8 × Intel® Core™ i7-8565U CPU @ 1.80GHz
Memory: 15,5 GiB of RAM
Graphics Processor: Mesa Intel® UHD Graphics 620

Reproduce the issue in the Song Editor by selecting a project part and pressing Shift with the arrow key. Compare its selection behavior with the piano roll, where the expected behavior already works. Done means Shift plus the arrow keys extends the selection consistently in the Song Editor.

Written by the indexing model from the issue text.