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

Creating the .depend file requires the config.h file to exist and
suppress make's complaints if the .depend file doesn't exist when
initially parsing the makefile.

Old snprintf calls could have overflowed the buffers.

- Key mappings +/- are now general commands
- Use JPG as thumbnail cache file format instead of PNG
- Fixes issue #161

- Only load the thumbnails that are currently visible in the window
- Unload thumbnails that are leaving the visible area
- Much less memory needed, but scrolling is now slower
- This also unintentionally fixes issue #86