allow customisation of mouse buttons' actions

mouse buttons and other input buttons (tablet buttons for instance) should be remappable. actions assignable should include their existing functions (primary action, "push" page and context menu) plus secondary action (what you currently get with a shift-click), tertiary action (control-click), possibly other sub-actions currently accessed via keyboard modifiers and zoom while button is held. obviously others would think of more possible mappings.

this would improve accessibility for people used to other actions performed with mouse buttons (i am a Xara user and ex-Artworks user -- Artworks uses the right button to do what shift-click does in Inkscape, and Xara includes an option to set the right mouse button to shift-click) and also has the potential to improve efficiency for users who never use context menus (like me).