osd: fixed slider control not working on video osd

The slider control is not working if it's not included within the
VideoOSD. This PR refactor the slider control into an include and use
it on DialogSeekBar and VideoOSD. Now you can use the slider control to
seek as expected.
osd: invert play/pause button

This PR inverts the play/pause button, so the button indicates the
action which is triggered instead of the current playing state.
This fits to the normal behavior in most players on android.
Merge pull request #1 from xhaggi/seeking-slider

osd: fixed slider control not working on video osd
Merge pull request #2 from xhaggi/play-pause

osd: invert play/pause button