Replace eggaccelerator.c/h with native GTK+ code

[lanoxx]

Unless I missed something, all the code in eggaccelerator.c/h can be removed and replaced by gtk_accelerator or gdk_keymap functions.

Essentially at any place where a EggVirtualModifierType is currently used it should be possible to replace that by a GdkModifierType value, since eventually all occurences of a EggVirtualModifierType are mapped to and from a GdkModifierType. Once that is done it should be easier to identify places where we can drop or replace functions from eggaccelerator.c/h

[lanoxx]

Fixed by recent commits:

• keybindings: remove unused code (5af84cd0a8ce1a4b6bcf95874573e62a016ba00d)
• keybindings: use gtk to parse accelerators (6b1c1eeac497b3c5232355693531575fe8692e82)

The remaining code cannot be removed because GTK+ does not allow us to discover where scroll lock is bound.