Remove HeaderButton from mgwt and forward to ImageButtons.

Fixes #61
Add explicit class to all entries of a CellList.

Fixes issue #87
Merge pull request #142 from dankurka/remove_header_button

Remove HeaderButton from mgwt and forward to ImageButtons.
Change boundary for changing pages in Carousel.

Fixes #18
Merge pull request #143 from dankurka/fix_87

Add explicit class to all entries of a CellList.
Simplify checkbox css.

Fixes issue #59
Merge pull request #144 from dankurka/carousel_boundary

Change boundary for changing pages in Carousel.
Upgrade form & input elements to use newer flex box model.

Fixes #62 #60
Merge pull request #145 from dankurka/form_css_flex_box

Upgrade form & input elements to use newer flex box model.
Merge pull request #147 from dankurka/simplify_checkbox

Simplify checkbox css.
Update listbox.css to newer flex box model.

Fixes #58
Merge pull request #148 from dankurka/fix_listbox_css

Update listbox.css to newer flex box model.