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
New plugin: Pair Tag Highlighter

The plugin finds and highlights matching opening/closing
HTML tag by clicking or moving cursor inside a tag.
geanyinsertnum - compatibility with gtk+ 3
Extending HACKING-file with some information about e.g. build system (autotools)
Merge pull request #99 from vmkononenko/pairtaghighlighter

New plugin: Pair Tag Highlighter
Adding pairtaghighlighter to MAINTAINERS-file
Adding pairtaghighlighter plugin to list of translatable files
Improve GP_ARG_DISABLE() default to auto inside HACKING