Improve alignment and user interaction

This PR:
 - Add a css classs `ga-icon` to harmonize the size and alignment of buttons between components (catalogtree and layermanager)
 - Add an `:active`state to the buttons

Test [here](http://mf-geoadmin3.dev.bgdi.ch/fix_css/?X=190000.00&Y=660000.00&zoom=1&lang=fr&topic=luftbilder&bgLayer=ch.swisstopo.pixelkarte-grau&catalogNodes=1179,1180,1186&layers=ch.swisstopo.lubis-luftbilder_schwarzweiss,ch.swisstopo.lubis-luftbilder_farbe&layers_timestamp=99991231,99991231)

Test on:
 desktop -> FF, IE 9&10, chrome
  iPad iOS7&8 -> Safari, Chrome
                             Android -> Default, Chrome, FF
                            Surface -> IE 11