Remove vendor prefixed properties from CSS based on 1.5 browser support

[jaspermdegroot]

[jaspermdegroot]

We are going to add a Grunt task for this and remove the vendor prefixes from the CSS files.

[jaspermdegroot]

After merging PR #8500 the following things remain to be done:

• remove superfluous vendor prefixes theme CSS (can't use autoprefixer here because we need the inline comments for the ThemeRoller)
• remove filter: Alpha(Opacity=*); (we will only support IE9+)
• remove .ui-toolbar-android-2x-fixed rule in fixed toolbar CSS (we will drop support for Android 2.3) (note: remove the JS that adds the class as well)
• remove .ui-popup-arrow-container.ie .ui-popup-arrow from popup arrow CSS (we will only support IE9+)

[Bachstelze]

Can we move the further removements to the next milestone?