[gtk3] Incr/Decr Transparency shortcut conflicts with VTE's scrolling

The default shortcut for Increase/Decrease Transparency are Control+Shift+Up/Down.

VTE uses these keypresses (built-in) for scrolling vertically by a single line, which many people might find useful. Due to Guake grabbing these key combos, they don't reach VTE now.

Suggestion: maybe change these to Ctrl+Shift+Left/Right.

(Nit: Since the feature's name talks about "transparency" rather than "opacity", the "reverse" setting seems a bit more intuitive to me. That is, Ctrl+Shift+Left decreases opacity, aka increases transparency, and C+S+Right decreases transparency. But maybe it's just me.)