Add seek bar for player

BUG:
parent widget's paint engine must be qpainter, otherwise children may
not update
TODO:
subclass of QSlider. press slider can not change value
auto hide slider
Merge branch 'master' into player
player: add simple control buttons
Merge branch 'master' into player
emit stopped() only once to avoid emitting after a new play started
use slider from SMPlayer to support seek on press
player: add button class
player: button icon changes if state changes
add missing icons
player button
player: add time and title label etc
player: show window as early as possible
player: allow label not show complete text
player: emit signal after connections ready
player: add capture button etc.

FIXME:
QLabel size is too large to show the text
misc. emit stopped once
player: hide widgets except renderer if fullscreen
player: add menu button
Merge branch 'player'