Remove the icons from avogadroapp (moved to avogadrolibs/qtgui)

Also use the new standardIcon utility function to ensure that fallbacks
are handled correctly on platforms that do not support icon theming.

Change-Id: I2dfabef6edcb18433d25e2adc501c2d826c9f57b
Merge remote-tracking branch 'origin/master' into copy-paste-icons

Conflicts:
	avogadro/mainwindow.cpp

Change-Id: I8f004425264d1c856722b1167511c28b7e3b6f83
Merge topic 'copy-paste-icons' into master

0ebe6307 Merge remote-tracking branch 'origin/master' into copy-paste-icons
2d4f38a5 Remove the icons from avogadroapp (moved to avogadrolibs/qtgui)