improve the names of the vcs icons

adopt a format like "vcs-<action>-<vcstype>-<vendor>",
so there is no clash with other potential icons, and at the same time it could open the door to possible generic (non-vendor) icon names

also, move them to the hicolor theme, so they will show up also when using an icon theme different than oxygen (but keeping the possibility to be overridden by the current theme)