Fix createTab incognito and createTab window commands with no URLs on Firefox

[nsunderland1]

Description

Currently, user-defined mappings for createTab incognito and createTab window with no URL parameters fail on Firefox, e.g.:

map X createTab incognito
map X createTab window

This is because these commands try to call chrome.windows.create with about:newtab as a URL parameter, which is illegal in Firefox. The solution is to instead call chrome.windows.create with the URL parameter omitted, which makes it default to the Firefox's home page (for createTab window) or about:privatebrowsing (for createTab incognito). A similar solution is already in place for the basic createTab command:

vimium/background_scripts/main.js

Lines 181 to 183 in bdf654a

	// Firefox does not support "about:newtab" in chrome.tabs.create.
	if (winConfig["url"] === Settings.defaults.newTabUrl)
	delete winConfig["url"];

[philc]

Thank you @nsunderland1 for writing up a clear solution. I've adapted your PR to the latest code base and this is now fixed. It will ship with Vimium v2.0.

[philc]

In my testing, this step was unnecessary -- if windowConfig.url was an empty array, the new window still opened as expected.