feat: Remember the last read-only state of each database

The app has supported a global setting for opening (all) databases in
read-only mode. But that's not particularly flexible for the use case
where you have one database that should be read-only and one that should
be read-write.

Previously, to handle this use case you could open one database in
read-only mode, but the next time you attempted to open the same
database, it would "forget" that, so you would have to toggle it to
read-only mode again manually. This commit changes that behavior so that
if you toggle a database to read-only mode, it'll be remembered the next
time you open the database. (You can still toggle it back to read-write
if you change your mind, and that, too, will be remembered the next time
you open the database.)

database/src/main/java/com/kunzisoft/keepass/model/DatabaseFile.kt

@@ -6,6 +6,7 @@ import com.kunzisoft.keepass.hardware.HardwareKey
 data class DatabaseFile(var databaseUri: Uri? = null,
                         var keyFileUri: Uri? = null,
                         var hardwareKey: HardwareKey? = null,
+                        var readOnly: Boolean? = null,
                         var databaseDecodedPath: String? = null,
                         var databaseAlias: String? = null,
                         var databaseFileExists: Boolean = false,