bitbake: event/hob: Add a button for network tests in the proxy settings

bitbake: event/hob: Add a button for network tests in the proxy settings

This patch allows the user to check the network connectivity in
the "Proxy" page ("Settings" dialog) by adding a button which provides
this functionality. It also disables retrigerring sanity checks if the
proxy values are changed, since now the proxy checks are explicit.
Note that this patch depends on a patch in oe-core
("sanity.bbclass: trigger network tests explicitly"). It will
not work properly if the patch in oe-core is not merged.

[YOCTO #3026]

(Bitbake rev: cb1354d29c0be27aee57b9783c724457ef6725fb)

Signed-off-by: Bogdan Marinescu <bogdan.a.marinescu@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>