Add Ayin/Light theme.

Nothing's HDPI yet.
Merge branch 'light-theme'
Fix sidebar selection color in Ayin.

Issue: close #4, light theme.
Add icons for Ayin.

Damn global gitignore file made me skip these. Fixed.
Move dark images into assets/dark/
Remove old light files.
Add tab.psd
Update dark image paths.
Update dark tabs.
Update tab.psd
Add icon PSDs.
Remove (soon-to-be) unused images.
Add Ayin scrollbar to Nil as temp. texture.
Update Nil to same tab structure as Ayin.
Update scroll bars.

Dark theme is currently using Ayin's scrollbars since the dark version
isn't yet done.
Update arrow images.
Add HDPI arrow images.
Move arrow textures to assets/arrows/.

These directories are going to get crowded with 2x the images.
Move tabs into their own directory.

Pre-empting the ton of new files.
Add arrows.psd