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
fix search-field: display the cleaning cross when reopening the field

On dropdown or selects, when we opened the dropdown with a value already in the field,
the cleaning cross was not displayed unless we made a change inside it. By adding the
"search-filter--is-active" in the link function if the input already has a value, the bug is fixed.

fix second point of #26
fix text-field: move the label when init with a value

On text fields, the label was not moved up when the ng-model associated had an value
at the init of the components. Fixed it by adding the class if the ng-model has a value.
rework text-fields: use '&' binding instead of '@'
    
Using @ binding, we were able to manage expression to string. By replacing with a &,
we can still use expressions but don't need to manage angular templating anymore. And
we can have real boolean value.
This commit also update the demo to show more case and use real ng-model.

BREAKING: the text-fields parameter do not take a string anymore. They need boolean values.
feat search filter: add width and position attrs

New parameters for position, width of the search filter.
fix search filter: use filter-width parameter

Use filter-width parameter instead of width parameter.