window: Update slider position on change-value
Currently, the slider position is updated in response to the GstPlay player position update. Therefore, when the user seeks, the slider widget is updated only once playback is resumed. This results in a non-smooth UI/UX.
This commit add slider position update as part of the 'change-value' event handler. This means the position and label of the slider will be updated as soon as the user seeks, and even while the user is seeking.
Tested on RK3399 Rockpi4 board, on Fedora Rawhide running Weston compositor.