mythburn.py: only use toprettyxml() in Python >= 2.7.3

There is a bug in older versions of that function which inserts additional
white space into text nodes causing problems when reading back in the
prettified xml file.

(cherry picked from commit 41264b6006221900d0a5fa685f67e6b6c0e972ef)
mythburn.py: fix another utcisoformat() missed in fa94c154c7

(cherry picked from commit 90cdee0efa51bdc8ecb20fc1dadfe676a1e1fddc)
MythZoneMinder: use the itemVisible signal to update the event images

This makes scrolling in the grid a little quicker since it's now only
updating the images for the visible items.

(cherry picked from commit 21296a9fdcb672a21d884d04deb5983addebaf8d)
mythzmserver: fix a possible segfault if no event frames are found

(cherry picked from commit a33c327a1bc7cde68bbb2dfc7648f09b7dc2fdf8)