The common compiler and linker flags control, among other things, the
default visibility of symbols; without them, we leak symbols that ought
to be private.
We're mixing a lot of styles in the Meson build files. This is an
attempt at making everything slightly more consistent in terms of
whitespace and indentation.
This is how it's done in the autotools build. Also avoids problems
with multiple source files having the same name (gdkeventsource.c).
Also move broadway backend code into broadway subdir.
