With this patch, toolbar tabs can be switched by pressing the underlined character of the tab on the keyboard while hovering over the toolbar.
K, D, I, E get avoided when assigning shortcuts. The screenshot is outdated.
The character to underline in a certain tab is currently automatically found. Another way to do this would be to scan for an ampersand in the toolbar category name. This however would introduce conflicts with multiple addons defining names with the same character as a shortcut.