Give after playback single time event precedence over loops.

Fixes #4978
Add "Play next file in the folder" event to single time events menu.

From my understanding people doesn't care about loops and even
persistence of the after playback switches, they just want to have it
easy accessible and executed on playlist end. This isn't fully consistent
with every time events which respect loop counter, but it doesn't have to
be, we ought to look at it as two separate features which provide
different possible workflows. This way majority of the users should be happy,
both this who wants to use looping feature and those who wants to abuse
"play next" feature.

Fixes #4971
Position context menus below selected item or in top left corner of the
window when they are triggered by App key.

Fixes #4995
Document all command line switches.

We have better dialog now that allow more entries to be shown.
Specify in OSD message that after playback event has been disabled.
Do not adjust window size for video frame aspect ratio when video size
is independent from window size.

This fixes the case when scaled video frame size is bigger than work
area and we want to have bigger viewport.