Fix #9847: always fall back to file timestamp
(cherry picked from commit f33d31898155be7ad1d41a29b561590d27e676b5)
Fix some memleaks from dt_conf_get_string()

That function returns a copy of the string that has to be g_free()'ed!
(cherry picked from commit fe8b0890010cc4d3d685ec36e1d1d33c11db0907)

Conflicts:
	src/gui/camera_import_dialog.c
	src/libs/capture.c
	src/views/capture.c
	src/views/lighttable.c
	src/views/slideshow.c
Write to stderr when browser couldn't be opened
(cherry picked from commit 01da945808481c3737a06d4c42ed3878a219dda6)
Some more leaks: result of g_path_get_basename should be free'd
(cherry picked from commit 18c4b5865a3c8b4613e8c3c7ffddbe67a90da41b)
Experimental fix for crash(?) on dt exit, found by pmjdebru1jn
(cherry picked from commit e7eac4fb3d9e13f7604cc68ef616de28b01b9523)