Read setting to emulate Numpad

[HugoSlt]

Godot version:
3.1, 890fc10

Issue description:
On laptops it is particularly handy to be able to emulate the numpad. This option was proposed in #3612 and the issue was closed.

Yet on the current master, in editor/editor_settings.cpp the line 448 is commented :

// _initial_set("editors/3d/navigation/emulate_numpad", false); not used at the moment

Commit f328a35 commented this line. If the implementation of emulate numpad is working I think we should re-enable it for 3.1.

Steps to reproduce:
Go to editor settings, look for emulate numpad in editors -> 3d. The option is not here.

[akien-mga]

There's no more code for emulating numpad in the current master branch:

$ rg "emulate_numpad"
editor/editor_settings.cpp
448:    // _initial_set("editors/3d/navigation/emulate_numpad", false); not used at the moment

so #3689 was undone and would have to be reimplemented.

[clayjohn]

Feature and improvement proposals for the Godot Engine are now being discussed and reviewed in a dedicated Godot Improvement Proposals (GIP) (godotengine/godot-proposals) issue tracker. The GIP tracker has a detailed issue template designed so that proposals include all the relevant information to start a productive discussion and help the community assess the validity of the proposal for the engine.

The main (godotengine/godot) tracker is now solely dedicated to bug reports and Pull Requests, enabling contributors to have a better focus on bug fixing work. Therefore, we are now closing all older feature proposals on the main issue tracker.

If you are interested in this feature proposal, please open a new proposal on the GIP tracker following the given issue template (after checking that it doesn't exist already). Be sure to reference this closed issue if it includes any relevant discussion (which you are also encouraged to summarize in the new proposal). Thanks in advance!