usercore: remove duplicate code in ItemHandle

createDesktopShortcut and createMenuEntry are doing the same, but are
installing the created file with other commands. Move the code into
a private method
usercore: make createDesktopfile a private ItemHandle method
Merge pull request #561 from karolherbst/usercore_desktop

usercore: remove code duplication in desktop file creation
static/scriptengine: use Handle instead of Persistence

fixes #569

v2: make it compatible with older versions of v8
Merge pull request #570 from karolherbst/bug569

static/scriptengine: use Handle instead of Persistence