Use xmodmap to map Search+key bindings

This branch adds support for Chrome OS-like key bindings (easily checked in Chrome OS by typing Ctrl+Alt+Shift+/). That allows to type Page Up/Down/Home/End/Delete, etc.

This is accomplished by remapping the Search key to Mode_switch first, then remapping keys.

Issues:
 - Search+. mapping to Insert would break non-US keyboards
 - Are the bindings specific to Samsung ARM Chromebook? Does the Search key always corresponds keycode 0x85.
 - Other shortcuts like Ctrl+Alt+Arrows cannot be mapped easily (I tried several ways, without success)