Use symbolic icons in toolbar

[haarp]

Hello,

Viewnior uses a few icons in its toolbar that can, with certain GTK and icon themes result in unreadable icons, such as dark-on-dark or bright-on-bright.

"Just" using bright icons on dark themes is not a good solution, as this a) Not all backgrounds are necessarily dark, and b) There will be many occasions where the bright icons will look out of place in other places/applications.

The proper way to solve this is to use symbolic icons, which are monochrome icons that will adopt the font color. Themes will set a foreground font color that provides a good contrast to the background. Having icons follow this is a clever solution.

Otherwise, users will invariably run into unreadable theme/icon combinations.

This is a sequel to #76

Thanks!