Add ModeDialer

[sspanak]

Extend Mode123 with a new ModeDialer. This will further reduce the usage of mEditing and hopefully resolve punctuation typing problems in 123 mode using the on-screen keyboard. Also, this will make it easier to fix #246.

• Move hold-0-to-type-plus in ModeDialer. Not a good idea. Decided to keep it in 123 after all.
• In TraditionalT9 class, replace all usages of mEditing == DIALER with a check for the new InputMode
• Both modes must have isNumeric(), replacing the current is123(), in order to replace mEditing == STRICT_NUMERIC.