Fix typo in the e_rgba_to_color documentation
EHTMLEditorView - When switching from HTML mode to the Plain text mode remove all image backgrounds that are set
EColorCombo - Improve the widget

Take into account the full transparent color (alpha channel is 0). When it
is set render the current color differently (gray and white squares).
EHTMLEditorSelection - Try to unref the node when it is set
Improve and fix various dialogs.

In EHTMLEditorCellDialog and EHTMLEditorTableDialog encode the
background images for elements into the base64 format to prevent
from deleting them when sending the HTML message. Also set the default
background color in these dialogs to transparent. In dialogs previously
mentioned and as well as in EHTMLEditorPageDialog add the button to
remove the previously set background image.