Maybe keybindings should allow mouse buttons > 3 to be bound.
That might be interesting to incorporate it somehow into the "key" bindings. I think something more elaborate could be looked at later and shouldn't block this PR.
@Yanpas what if you just added a single "Various" preference that is enabled by default like `mouse-back-forward-enabled` that allowed this to be disabled. My main concern is that on some device, platform or with certain X mappings, this could make Geany unusable. The "Various" preference would provide an escape hatch for this case.