xkbd 0.8.15: Add default common slides.

xkbd 0.8.15: Add default common slides.
* xkbd has supports for "slides", where instead of just click on key,
there's a gesture up, down, left, or right. These can be specified in
keyboard definition, and xkbd has defaults for each 4. But of 4 pristine
defaults, only 1 is useful - up is shift+key. The other are for ctrl, alt,
etc modifiers, which are less useful in real life. Instead, have following
defaults: left - backspace, right - space, down - Enter. This means that
on common xkbd file w/o slide definition, making a right-to-left gesture
anywhere on keyboard will be equivalent of backspace, etc.