Add IE11 Support for pasting images in the composer

[Falco]

And we can begin to ditch the Firefox hack when 50+ gets enough market share:

Firefox 50 makes image pasting hack superfluous

There is still some time until Firefox 50 becomes stable in November 2016, but as a noteworthy number of developers could use it already from September when it will promoted to Firefox Developer Edition (in fact, I’m using it already with Firefox Nightly), I suppose it is not too early to point out that the following firefoxPastingHack for pasting images will not be necessary (and even counterproductive) any more due to clipboardData.items being implemented in Firefox 50 (see Mozilla Bug 906420)…