Fixes #39 Video assets

This is an asset PR, copying existing @2x files to video app.
We still need the following 2x assets after this PR:

videos/images/button_bg_normal.png
videos/images/button_bg_pressed.png
videos/images/scroll_bar_bg.png
videos/images/scrubber_pressed.png
videos/images/scrubber.png
videos/images/timeline_filled.png
videos/images/timeline.png
videos/images/portrait/button_bg_normal.png
videos/images/portrait/button_bg_pressed.png
videos/images/portrait/scroll_bar_bg.png

of which the following are assets also used in music app:
scroll_bar_bg.png
scrubber_pressed.png
scrubber.png
timeline_filled.png
timeline.png