Code review comment for lp://staging/~phurley/indicator-session/lp-957563

Seems far more complex and prone to omissions/special cases that another merge fixing the same issue: https://code.launchpad.net/~ballogy/indicator-session/better-fallback-icon/+merge/141608 (which however doesn't take care of retaining the fix for bug 1048348).

I'm sure there's a way to simplify this to not require a list of icons hard-coded in indicator-session -- perhaps by using the g_themed_icon_with_default_fallbacks mechanisms, and checking if it results in a valid icon, and dealing with when it doesn't?