multiplatform: split common/android/desktop (#2672)

* multiplatform: relocated code to its new place

* code becomes better

* renamed file

* fixes for BASE64 and images, and changes for appFileUri

* different Base64 for both platforms

* fix file saving on long click

* platformCallbacks refactoring

* renamed callbacks to platform

* eol

---------

Co-authored-by: Evgeny Poberezkin <2769109+epoberezkin@users.noreply.github.com>

apps/multiplatform/desktop/src/jvmMain/kotlin/Main.kt

@@ -0,0 +1,8 @@
+import chat.simplex.common.platform.*
+import chat.simplex.common.showApp
+
+fun main() {
+  initHaskell()
+  initApp()
+  return showApp()
+}