[RFC] Protect Player!

When I was looking at the action handling stuff re. the ACTION_PREV/NEXT_ITEM (other pull request) I came across the very many calls to the player from different places, and thought it wasn't very clean.

The idea is to remove public access to CApplication::m_pPlayer, by making it protected and adding a const getter.
Additional benefits are:
- remove unnecessary null-checks e.g. when also checking IsPlayingXXX 
- calling CApplication::IsPlaying / IsPaused instead of player's equivalents

No functionality is improved, but code looks cleaner. 
This is RFC only, so happy to drop if you think this is BS.