feat tabs: remove padding on .tabs__panes

Before there was a 24px padding on .tabs__panes. We decided to remove
it in order to allow users to have more flexibility on tabs content.
chore demo: tabs demo updated without padding

We removed padding on .tabs__panes. Instead, we added helpers classes
on content tabs paragraphs.
feat tabs: tabs can have a shadow

Tabs can now have a shadow with a new parameter named z-depth.
chore demo: tabs demo updated, example with tabs shadow

The tabs demo has been updated with an example of tabs with a z-depth
parameter set to ‘‘1’’ . This parameter accepts 3 values : 1, 2 and 3.
Merge pull request #29 from lumapps/feat/tabs

Feat/tabs : remove padding and add shadow feature to tabs