fix file upload in overlays

To include files in our upload this uses the FormData API.

Unfortunately, FormData is only available in IE 10+. In
older IEs, the whole form will be unusable.

To limit the damage this does only use FormData where necessary.
use FormData whenever available

This is an improvement because now forms with
enctype="multipart/form-data" will non break completely
on non-supporting browsers
add warning to ajax form submit for old browsers
abstract overlay.close
close overlays on esc (fixes #598)
rm js tooltips (fixes #208)

The JavaScript tooltips we are using are buggy.

I see no real reason why we use the in the first place
because the tooltip implementation in browsers is quite good
and probably has benefits in terms of accessibility, too.

So this removes the js tooltips.
Merge pull request #729 from xi/feature-598-fix-overlay-esc

fix overlay esc
Merge pull request #731 from xi/feature-208-rm-js-tooltips

rm js tooltips (fixes #208)
use overlay's native close()
Merge pull request #728 from xi/feature-724-fix-file-upload-in-overlay

fix file upload in overlays
add instance_option page_index_as_tiles
add page logo
config ui for page logos
enforce page_index_as_tiles
add note on page image upload form