Better shortcut system that avoids conflicts and saturation

This issue is opened to collect our shortcut problems and track any potential progress in addressing them.

Currently, we have two shortcuts:

• Ctrl+Shift+L for new circuit and
• Ctrl+Shift+U for new identity.

Shortcuts currently have the following problems:

• Ours can conflict with Firefox's. E.g. see #43602 (closed).
• Firefox's shortcuts are set up so they can vary with locales, which can also lead to conflicts. Although, in practice the shortcuts seem to be the same for all locales.
• The list of shortcuts is saturated. Between firefox and desktops, the "Ctrl" and "Ctrl+Shift" shortcuts are mostly filled. This means that new shortcuts from firefox are "Ctrl+Alt".
• They can conflict with desktop shortcuts. E.g. #40949 (closed).