Implement the possibility of assigning a mouse button as a modifier in keymap preferences.

I´ve worked for several years with CAD programs like CATIA and NX. In those programs, viewport navigation is performed by using only the mouse, without needing to touch the keyboard. The way it works is by combining mouse button presses to perform the different moves (pan, rotate, zoom).

Right now I can´t find a way to assign a two mouse button combination to a certain action, only keyboard + mouse or a single mouse button. It would be nice to be able to combine two mouse button presses. It may be achieved by allowing mouse buttons as modifiers in the keymap preferences.

This would allow to use only the mouse to perform common tasks, for example viewport navigation. Furthermore, this would be even more useful for people with more than 3 mouse buttons, as they may be able to perform common actions like move, scale, etc. just by combining mouse button presses, without having to touch the keyboard.

Thank you for the great job you are doing!