Improve meson build.

-Add all required packages as wrap dependencies
-Move source lsitnings to separate files
-Proper dependencies declarations
-Set global D compiler arguments
-Add few asserts
-Add test exe
-Other small improvements

Tested with dmd with betterC, demos executable has to be run from 'demos' directory to properly load assets.

subprojects/cimgui.wrap

@@ -0,0 +1,8 @@
+[wrap-git]
+url = https://github.com/cimgui/cimgui.git
+revision = 1c65ee2bdc719fb3ef62b4615d66fe8effa21148
+clone-recursive = true
+patch_directory = cimgui
+
+[provide]
+dependency_names = cimgui