Allow edge keys instead of corner keys (swipe vertically/horizontally)

[MaxGyver83]

Fix/implement #56.

Add a new boolean parameter "edgekeys" for defining keys that have the additional (swipe) keys on the edges (top, right, left, bottom) instead of at the corners (top left, top right, bottom left, bottom right).

Example:

<key width="1.2" key1="up" key2="right" key3="left" key4="down" edgekeys="true"/>

If the edgekeys parameter is omitted, it defaults to false (swipe diagonally for additional keys).

This pull request also changes the arrow keys in all layouts to the alternative edge style:

Before:

After:

[Julow]

I've used this since yesterday and it's working great :) I do prefer the arrows now, though I don't use them much.
Thanks a lot !

@avently mentioned that the arrow keys were already fine but I think vertical/horizontal movements are easier than diagonals at the bottom of the screen in addition to being more intuitive. I'm also investigating improving cursor movements and this will help a lot.