Enable caching of YouTube data API v3 requests

This patch is a small overhaul of network request caching subsystem.
Besides allowing YouTube data API v3 responses to be cached it also
exposes cache customization to the user via application settings.
YTVideoList: Add BusyIndicator to the element

Common enough feature so move it into the element itself. No need to
have each page using YTVideoList duplicating the feature.
MainMenu: Update icons for 0.4.x release

I did not really like some of the 0.3.x icons, so replace them with some
alternatives.
ChannelBrowser: Make publish date / video count positining more flexible

Instead of hard coding the amount of space for each field to take 2/3
and 1/3 width of the parent respectively make the layout more flexible.