Refactor main.js, enable custom screensize

[pciavald]

This is based off a merged #1610 as it was already modifying how the url is handled in main.js. Probably easier to look directly at main.js and helper/electron.js, this is where most of the change is.

I've extracted most of the electron-related logic into electron.js and grouped together behaviors that were a bit scattered in main.js. The logic is more state based and with the default values for BrowserWindow and state overrides, it was easy to implement custom window properties from the user config. As for the previous PR i've set everything to default to null so nothing gets overridden by default, but user can choose a different behavior.

As an example, we use it with these settings:

{
	"config": {
		"octodash": {
			"window": {
				"width": 800,
				"height": 454,
				"y": 26,
				"fullscreen": false
			}
		}
	}
}

x is also exposed but we don't need it.

If you prefer that these are written by default in the config file, i can do it based on what's currently happening. Feel free to close if you don't want it in upstream.

The files diff will get much clearer when i18n is merged.

Fixes #1586