None of the mouse mappings uses a keyboard modifier, making it possible to
access the most basic features by only using the mouse.

Next/previous image with left button depending on cursor position, middle
button for dragging, right button for switching to thumnail mode and wheel for
zooming.

Users can keep the old behaviour by simply not adapting the changes to the
buttons array in config.def.h to their config.h file.

This makes mouse panning more direct and faster.

Verbose when build with `V=1'.

Fixes issue #167. Long after closing this issue (kind of wontfix) we have switched to
GNU make and can thus use its features like VPATH.

Misbehaving command lines in the key-handler that rewrite the given files and
thus replace symbolic links instead of their targets shall call realpath(1).

Fixes issue #205 and reverts commit 92e3b578.

Related to issue #253, which originally asked for a non-blocking key-handler.

Use PNG if image has alpha layer, JPG if not. Fixes issue #209

Programs called by the key handler might replace symlinks instead of their
targets.

According to the official [Icon Theme Spec](http://standards.freedesktop.org/icon-theme-spec/icon-theme-spec-latest.html#icon_lookup) the Icon key in the desktop entry file should not contain the file extension.

- Use non-POSIX :=,?=,+= macro assignments, fixes issue #97
- No optimization level set in CFLAGS, expected to be set in environment
- Automatic header dependency tracking with one .d file per compilation unit
- Better fix for issue #181