Fixups for previous commit

- remove unneeded import
- rename variables
Fix icon reference by position instead of key

Introduced by mistake in 18bc0aac .
Activity palette: remove space in label Description
Move all initial configuration to activity.setup

And set colors to the icon at the top of the activity palette.
Fix palette variable from global to local inside palette module

All these should be declared with 'var', otherwise they are global.
Improve ActivityPalette API
Improve width of palettes

- minimun width of 3 cells
- maximun width of 5 cells
Style palette rows and activity palette
Fix button position when mixing buttons with and without palettes

The CSS :before selector is used to add the little arrow in the
button.  This works fine for the current tests, but an important test
was missing: mix buttons with and without palettes.  The mix provoked
a displacement of the buttons with palettes.

This commit fixes that by adding :before to all buttons to make them
have all the same structure in the DOM.  And then adding the arrow
graphic only to the ones with palettes.