Revert "Drop support for glib < 2.36"

This reverts commit 495f56659d607938db00e34c9f84bac24013d999.
Drop support for Glib <2.32
Revert "Drop support for Glib <2.32"

This reverts commit e9a0c53bdb5755d903e1f2ad90e47b8c373c3995.
Revert "Revert "Drop support for glib < 2.36""

This reverts commit 13392e50c61ce3f3d7d04b59677380f935cf3d3c.
use proper GTK+ version in pluma.pc file
zero-initialize all signals arrays
don't leak memory
plugins: don't leak memory