Fix app icon install destionation

QIcon::fromTheme() doesn't search in /usr/share/icons. And it shouldn't.
It also don't search in /usr/share/pixmaps and it should. The proper
solution is to install the icon to the default theme (hicolor). This
way all the icon engines can find it.
Merge pull request #14 from luis-pereira/fix-icon-install-dir

Fix app icon install destionation