Remove DefaultMongolTextEditingShortcuts from MongolEditableTextState

[Satsrag]

• Remove DefaultMongolTextEditingShortcuts to prevent it intercept keyboard event

• If developers using this library need to switch up/down and left/right keys, insert DefaultMongolTextEditingShortcuts to WidgetsApp's builder. Use one of these.

  WidgetsApp(
    ...
    builder: (context, child) {
      return DefaultMongolTextEditingShortcuts(child: child);
    },
    ...
  )

  MaterialApp(
    ...
    builder: (context, child) {
      return DefaultMongolTextEditingShortcuts(child: child);
    },
    ...
  )

  CupertinoApp(
    ...
    builder: (context, child) {
      return DefaultMongolTextEditingShortcuts(child: child);
    },
    ...
  )

This is the second solution that is mentioned in #33. The last solution is not working. Such as, in TextField, left/right+alt and up/down+meta will invoke ExtendSelectionToLineBreakIntent and up/down+alt will invoke ExtendSelectionToNextWordBoundaryIntent. In MongolTextField we will switch ExtendSelectionToLineBreakIntent and ExtendSelectionToNextWordBoundaryIntent to switch left/right+alt and up/down+alt. However, ExtendSelectionToLineBreakIntent also means pressing up/down+meta. This is a conflict and solving it is complex.

[suragch]

Can we change the name from DefaultMongolTextEditingShortcuts to MongolTextEditingShortcuts? The reason is that it's not really the default. It's a required variation from the default that the developer must specify. The actual default is still DefaultTextEditingShortcuts.

This requested change affects the class name and file name.

[Satsrag]

You are right, I have renamed it.