Can't change Alt+[ or Alt+] shortcut (can't use on German keyboard)
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#### Steps to reproduce:
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- open Inkscape
- open shortcut settings
- try to change Alt+[ shortcut (rotate by 1 px)
#### What happened?
The shortcut is missing. [ (rotate step set in preferences) can be changed, and also Ctrl+[ (rotate 90°). They are defined as verbs.
#### What should have happened?
I should be able to change the shortcut.
Note: The Alt+] shortcut physically doesn't exist on German MacOS keyboards (https://gitlab.com/inkscape/inbox/-/issues/1000).
The shortcut is defined here:
* for sets of node selections: https://gitlab.com/inkscape/inkscape/-/blob/master/src/ui/tool/control-point-selection.cpp#L582
* for general selections (select tool): https://gitlab.com/inkscape/inkscape/-/blob/master/src/ui/tools/select-tool.cpp#L1043
#### Inkscape Version and Operating System:
- Inkscape Version: 1.0 (5b275a35d9, 2020-05-03) snap
- Operating System: Ubuntu
- Operating System version: 20.04
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