Input: Fix multibutton input for mouse interaction

[Megamouse]

• Adds tooltips for button mapping (hover on a button).
• Allows for updates on already pressed keyboard buttons when pressing the pressure sensitivity button.
• Should hopefully fix all issues with multibuttons when using the keyboard.

Previously, the mouse presses, mouse wheel and mouse move would override any other key on the same button.
Additionally, mouse interaction was not working in tandem at all anyway.
Last but not least this should also fix any issues regarding stick or button interpolation with multibuttons on the keyboard.