Fix icon in about dialog (all platforms) and application window icon for linux non-GNOME

Before this patch not-installed meld or meld built for windows has problems loading its own svg icon, since the icon doesn't present in resources and were never installed into the OS.

This leads to lack of application icon on non-Gnome linux DE's and lack of icon inside about window on all platforms.

This patch embeds application icon into resources, so it is usable from inside meld even if it is not installed.

This change can lead to a situation when builtin icon overrides the icon from a user-configured icon theme.

About problem reproduction: actually the absence of the icon in the about window reproduces on all my platforms and machines. So I can't say what should be done to reproduce the issue)

I made a git bisection and it pointed to the commit f6c59dcb

So, I suppose that the issue will NOT reproduce if an icon named org.gnome.Meld is contained in a system-wide icon store (for example meld is installed).