Improved progress state for e-button

[patric-eberle]

Currently the e-button component replaces it's content, when it switches to progress mode. This prevents us from using the v-t directive or any other directive, that places something inside the button, because it will be replaced with the progress. Would it instead be possible to use an absolute element for the progress state, that covers the main content? This would also prevent size changes when switching between the states.