Optionally quit or remove missing plugin completely from perspective

[lucasw]

Continues #109 and #110

I've started a new branch ab615d2 which uses a SimpleSettingsDialog to ask the user if they want to quit when encountering a missing plugin or continue without it. (If there are many missing plugins they'll have to click 'ok' that many times)

When they click continue the plugin doesn't appear, and if saved it properly isn't in the list of running-plugins in the .perspective (the rqt_gui.ini isn't as readable so I haven't found the equivalent there), but the settings for the missing plugin stay around.

The next time a perspective is loaded with the remnants of a missing plugin it appears without the plugin and no attempt to load it is made, but the left-overs persist- is it ugly but harmless cruft or corruption that might cause problems?

Better to remove it but I'm not sure how- I haven't traced what happens when a plugin is removed through the gui, maybe that could be used here? There is a function _remove_running_plugin but here the plugin isn't running yet so isn't in that list.