Make glade useable for everyone

@adrien.plazas brings up points against glade usage in calls!7 (comment 3139).

writes defaults for properties in UI files
adds useless placeholers
it doesn't support all the features of GtkBuilder, like properties bindings

@adrien.plazas could you please follow up with links to upstream bugs here? We need to get these fixed so that working on ui files with and without glade is possible. Otherwise the whole development story will suck.

(I'm a happy glade user though).

Edited Jul 27, 2018 by Guido Gunther

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