Description of the new feature/enhancement

Follow-up on #10801: the added browser_back and _forward keys are the keyboard keys, not the mouse buttons. It might be nice to add support for these buttons as well, usually they are VK_XBUTTON1 and VK_XBUTTON2.

Proposed technical implementation details (optional)

The implementation will be a bit more complicated than just adding the key definitions, as now the mouse handlers should also check for keychord bindings. My proposal would be to add this to TermControl::_PointerPressedHandler. This would limit the support to when hovering the terminal control, so that might not be ideal. I am not sure what is actually desirable here.

Things to consider:

• Add support for "just" XBUTTON1/2, or also for other buttons?
• What should the textual representation of these buttons be? Make them coincide with browser_back and _forward, or separate names mouse_back and _forward?
• How do mouses with more macro buttons operate?