RFE: port to gtk4 and gtksourceview-5

[kloczek]

Looks like with below patch sequeler can be compiled with gtksourceview-4

--- a/src/meson.build~     2019-04-07 18:31:49.000000000 +0100
+++ b/src/meson.build      2019-04-23 06:51:25.180368032 +0100
@@ -59,7 +59,7 @@
         dependency('libgda-5.0'),
         dependency('libssh2'),
         dependency('libsecret-1'),
-        dependency('gtksourceview-3.0'),
+        dependency('gtksourceview-4'),
         m_dep,
         linux_dep
     ],

however it would be nice to start port it to gtk4 and gtksourceview-5.