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.

- 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

- Buffer for window content is bigger than the window, minimizes artifacts
  when window is resized
- Back to using XSetWindowBackgroundPixmap() instead of XCopyArea(),
  no need to handle exposure events; X server can show gray background directly
  after resize event before sxiv redraws the window contents