Use a prefix key to execute the key handler

The default prefix key is C-x, and can be changed in config.def.h. The
first key pressed after the prefix key will be passed the external key
handler, unless the key is Escape, which is used to cancel the prefix.
Fix tabs and wording
Merged pull request #129
Unified X atom initialization
Check if window manager supports fullscreen; related to issue #128
Preserve panning when switching images; fixes issue #131
Removed fit-win-to-img command
Revised scale mode and zoom level handling

- Scale mode is not reset to default value upon image loading anymore
- New default key binding to change mode to scale-down
- Removed scale mode setting from config.h
- Removed -d command line option, as this is now the default at startup
Set scale mode at startup via argument to -s option