IOPs are now compliant with -Wdouble-promotion

http://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html#index-Wdouble-promotion-292
Do not force anyone to use -Wdouble-promotion
Merge branch 'double-promotion' of https://github.com/LebedevRI/darktable
Fix #9847: always fall back to file timestamp
Fix some memleaks from dt_conf_get_string()

That function returns a copy of the string that has to be g_free()'ed!
Write to stderr when browser couldn't be opened
Some more leaks: result of g_path_get_basename should be free'd
Experimental fix for crash(?) on dt exit, found by pmjdebru1jn