GSettings: verify path validity on constructors

Don't allow constructing GSettings objects with invalid paths.

https://bugzilla.gnome.org/show_bug.cgi?id=704802
gsettings-tool: Add --version into general usage

https://bugzilla.gnome.org/show_bug.cgi?id=707002
Revert "Use g_return_val_if_fail() for developer-only messages"

This reverts commit 00f0795a84d23f2e2654a86f8bd3a233c8af3771.

https://bugzilla.gnome.org/show_bug.cgi?id=569017