GProject: Fix GTK+3 support

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
Focus the file view when switching to sidebar using the keyboard

Thanks to Giuseppe Corbelli.
Remove unnecessary assert
Fix box inclusion order
geanyinsertnum - compatibility with gtk+ 3
Adding stubs for structure
Adding entries for build-subfolder, configure.ac and Makefile.am
Remove dashes from plugin names
Some improvements based on b4n comments
Some smaller improvements
Adding information about Makefile.am inside plugin-root and plugin-src folders
Adding a short hint about structure of plugin

Where to put what.