Mer: Fix start/stop VM icon handling

Do not use action.setIcon(icon.pixmap(size, ...)) to scale down the
icons - provide properly scaled PNGs instead.

Note: using two QIcon instances instead of single QIcon together with
QAction::checked property as the buttons unfortunately do not obey this
property.

Also make the icons a bit bigger - 24x24 instead of 24x20.

Redo with:

cp IN.png IN-COPY.png
convert IN-COPY.png -resize 24x24 -gravity center -background transparent -extent 32x32 OUT.png
convert IN-COPY.png -resize 48x48 -gravity center -background transparent -extent 64x64 OUT@2x.png