Gdk::Display: Remove obsolete TODO comments

* gdk/src/display.hg: gdk_display_set_device_hooks() was removed before it
entered a stable API.
Vfuncs are not wrapped because GdkDisplayClass is private.
Gdk::RGBA, Color: Fix set_hsl(), improve documentation

* gdk/src/color.ccg:
* gdk/src/rgba.ccg: Fix the conversion from HSL to RGB color in set_hsl().
* gdk/src/color.hg:
* gdk/src/rgba.hg: Add docs of set_hsv() and set_hsl(). Change 'percentage'
to 'fraction' in docs. (The range is not 0..100, it's 0..1.)
Bug #720258.