make system icons work on Windows (and Mac?)

[kaa-ching]

Commit 2a8938 (and likely a few more commits in the near future) add icons to various actions. I'm going to need this to make a decent toolbar for the level editor.

However, on Windows these icons are not present. And possibly not on MacOSX either. So in the slightly longer run, we'll need to make a hybrid of:

• System theme using QIcon::fromTheme()
• QStyle's standardIcon() function

Which are explained well in: http://mithatkonar.com/wiki/doku.php/qt/icons

I'd guess this means creating a separate class for generating/building icons and have that class use either Qt's default ones or the system default ones...

[kaa-ching]

commit 8f17c95 adds even more icons to actions.

[kaa-ching]

See also:

• http://standards.freedesktop.org/icon-naming-spec/icon-naming-spec-latest.html
• http://standards.freedesktop.org/icon-theme-spec/icon-theme-spec-latest.html