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:
1. Search icon in the active icon theme — if found, use it.
2. Search it in the icon theme that is specified in Krusader config — if found, use it.
3. Search it in Breeze or Oxygen (in case any of these installed) — if found, use it.
4. 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