fixed translation (some spelling, some inconsistencies, some bad
wording)
used other applications to determine common practice for some of the terms
Desktop icon theme support
Use icons from current desktop theme (Qt functionality)
Reformat icons.qrc into theme format and fallback to the provided theme
if no system theme present in main.cpp
Add more action icons according to the XDG Icon Naming Specification
Merge pull request #82 from ypnos/master

German translation improvements & desktop icon theme support