Refactored and unified icon rendering, implemented per-icon fallback logic
Merge branch 'fix-missing-icons'
The merged branch contains implementation of per-icon fallback logic:
- Search icon in the active icon theme — if found, use it.
- Search it in the icon theme that is specified in Krusader config — if found, use it.
- Search it in Breeze or Oxygen (in case any of these installed) — if found, use it.
- Otherwise use a default "icon-missing" icon bundled with Krusader.
FIXED: [ 372964 ] At least one Oxygen icon in Krusader is not correctly seen (using Kubuntu 16.04)
BUG: 372964
FIXED: [ 372966 ] Some icons are not seen using a plain Ubuntu (not Kubuntu) 16.04
BUG: 372966
CCBUG: 376699
CCBUG: 381676
CCBUG: 386554
CCBUG: 388691
CCBUG: 391899
Differential Revision: https://phabricator.kde.org/D10352