Fix RH #633169 again.
https://bugzilla.redhat.com/show_bug.cgi?id=633169

If CAMEL_DEBUG=webkit is activated.

Avoid possible crash if the BODY contains only text nodes and not any elements.
Normally this can't happen unless the user drag and drops (or inserts) some HTML
content that was not written in the composer and the composer mode is set to
HTML.

If pressing the Backspace key to move a quoted block one level up (up to
BODY) and block's format is 'Preformatted' then if the block is empty
change the format of the block to 'Normal' if moved out of the quoted content.

Unit test done by Milan Crha.

They were missing the end tag of the PRE element.

While the selected days in Recurrences tab of an appointment's
week(s) occurrence type were meant to be drawn with selected color
background, it wasn't the case, thus it could be hard to notice
on some themes, notably on a GNOME dark theme.

This had been filled downstream as:
https://bugzilla.redhat.com/show_bug.cgi?id=1424729

We have to be sure that the BR element is the only child of its parent
before trying to replace it and also limit the search area only around
the caret.

The 'length' is used for the 'for' cycle and also inside the 'for'.
Better to not clash on them, to not cause confusion.

Signatures with images from previous version of Evolution, where images
are referenced as file:///..., were not properly added to the message
on send, resulting in broken signature on the receiving side.

This had been reported downstream at:
https://bugzilla.redhat.com/show_bug.cgi?id=1416423