small update of the UI of the symbollayers widgets

Hi !

I worked a little bit on a (very) small polishing of the Symbol Layer GUI.

The idea is mainly to optimize space (widget growth) and to regularize spacings/sizes (for readability).

It's pure GUI work (only .ui files are modified), so I guess it's not affected by feature freeze ?

I hope you like it ! 

I'll try to push this a little bit further : 
- data defined properties window
- try to make the symbol layers tree a little bit more compact
- make the general opacity slider wider
...

But I am not sure to have time in the coming days, so it would be great if you could merge it soon (since .ui conflicts are a pain to resolve).

Here are before/after screenshots :
https://www.dropbox.com/sh/83dtto8funy3vlb/kbiOe3Bsyk?m