Add mouse navigation button support for actions

[FWest98]

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?

[zadjii-msft]

Team decision: We're cool with these being allowed as keys in actions _if they come through as keys in PreviewKeyDown, where keybindings are processed. In 1.11, we thought you might just be able to get away with vk(5) (for VK_XBUTTON1 for example).

However, our cursory investigation makes us think that these don't come through as keys. This might be better off being dealt with by #1553. Probably as a sub-task of the bigger feature over there.

Thanks!

[FWest98]

I tried myself by adding these VK_XBUTTON1 entries, but those are not registered through any KeyDown or similar methods. I think it would be much neater to combine it with the issue you linked indeed.