Fixes issues #378 and #374.

Fixes issue #372.

There is no more need for this after the removal of the special color handling
for fullscreen mode in commit 2886876.

Fixes issues #361 and #367

It looks to me like this is also worthy of ignoring, since it's
autogenerated at make time.

Warning generated on MacOS, reported in issue #350.

Related to PR #346.

Two colors are more than enough!

Also makes the color names in config.def.h constant again.

The file names are separated with newlines in the primary and with spaces in
the clipboard.

Fixes issue #331

Putting image info in the title predates the info bar; it no longer seems
necessary.

Fixes issue #318.

Use uppercase for externally defined macros and lowercase for macros defined in
the makefile.

Also simplify generation of version.h.

Since d8114e8 the file name is used for caching as-is without adding
.jpg at the end, but sxiv -c still expected old format causing it to
remove all fresh thumbnails.

This also sets *markidx* when using the right mouse button in thumbnail mode.

*Shift-m* feels much more natural for *mark_range* and *Ctrl-u* is decent
enough for *unmark_all*.

Sets the marked state of all images ranging from the latest marked/unmarked
image to the current image, to the state of that latest toggled image.

VERSION string between releases is last release suffixed with '+'.
Additionally, use output of git-describe instead of VERSION string, if it is
not empty.

Fixes issue #308.