ui/app: call main from Android and iOS

Android can only run c-shared libraries which means that every
Gio program must create its window and event loop from an init
function.

The same applies to iOS but for a more benign reason: the gio tool
builds programs in c-archive mode for iOS and links the binary with
a Objective-C driver.

Allow Gio programs to run off its main function by linking to and
invoking main even from Android libraries and iOS ditto.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

ui/app/datadir_android.go

@@ -20,8 +20,6 @@ func dataDir() (string, error) {
 	return dataPath, nil
 }
 
-//export setDataDir
-func setDataDir(cdir *C.char, len C.int) {
-	dir := C.GoStringN(cdir, len)
+func setDataDir(dir string) {
 	dataDirChan <- dir
 }