Replace binds for methods with lambda methods

[s0]

As per "Replace any binds for methods with lambda methods" in #13

Rather than use .bind() for class methods before calling them, we can actually use lambda properties as class methods that automatically bind to this (this way, the caller doesn't need to remember to call bind() before calling the function).

I also see that we have functions called bind_this that call .bind on all functions of an object and it's prototype chain, both in the main greeter code and some of the themes, but I can't find references to it anywhere? As far as i can tell, all these functions can be deleted, but didn't want to do that until you've confirmed @JezerM that they are indeed not used in some weird reflective way etc...

[JezerM]

I don't think this solution is either worthy or necessary, as this "does a lot" just to remove two bind functions. Also, I don't like how it looks xd

About theme_utils.bind_this, surely almost no theme needs it, but lightdm-webkit2-theme-glorious does use it. I could set it as a deprecated method and remove it on future versions~