Add support for mouse back and forward buttons in the editor

[MattRix]

Describe the project you are working on:

Making an indie game with Godot

Describe the problem or limitation you are having in your project:

The back and forward buttons on a mouse do not work on in the editor to go back and forward

Describe the feature / enhancement and how it helps to overcome the problem or limitation:

Supporting mouse back and forward buttons

Describe how your proposal will work, with code, pseudocode, mockups, and/or diagrams:

The back and forward mouse buttons would act just like pressing alt+left and alt+right do right now

If this enhancement will not be used often, can it be worked around with a few lines of script?:

Not that I know of? Maybe installing a program like autohotkey in the OS, I suppose.

Is there a reason why this should be core and not an add-on in the asset library?:

It is a core usability feature

[Calinou]

Duplicate of #954.