When editing, add a subtle table cell delimitation that doesn't interfere with the skin

Although WYSIWYG is supposed to mimic as much as possible the final view result, there are existing issues reports from users that have troubles identifying table cells on Flamingo skin. (see XWIKI-10541, XWIKI-10894, http://markmail.org/thread/vso2wbbp3j3u2cnb)

Things to consider:

• Additional cell delimitation should be available just in edit mode;
• Consider that are Themes like 'Flatly' that have no table borders;
• Consider that are Themes like 'Sandstone' that provide a custom border-color;
• Consider that Bootstrap provides special table classes, see http://getbootstrap.com/css/#tables ;
• Colibri skin styling should not be affected.

Screenshots:

• Skin differences in View mode: colibriTables_View.png vs. flamingoTables_View
• Adding a default table default cell delimitation: colibriWYSYWGAddTable2x2_Edit.png vs. flamingoWYSYWGAddTable2x2_Edit.png