Add a Text Input plugin

This plugin allows the user to use more complex input methods for text input, such as the Google's Handwriting Keyboard for Japanese. Text is composed in a text field and submitted by a button.

The mousepad plugin straight up doesn't work with these keyboards. I tried to modify it to accept composed input, but the best I could achieve was worse than buggy, repeated words, backspace deleting the wrong text, etc, so I gave up and made this instead.

The plugin activity has one large EditText and two buttons - Send and Send with RETURN. Send with RETURN is very handy for searching in web forms (e.g. jisho.org kanji search).

Emoji don't work, but they don't work with the MousePad plugin either, whose API I reused for sending text, so that bug is somewhere deeper (maybe in the desktop software). Emoji are always sent as ?? for me.

I used the mousepad API, so the desktop counterpart can remain unchanged; it works reasonably well.

Here's what it looks like, at the moment:

Screenshot

The return icon is new, the arrow icon is borrowed from the media controls Play button. Maybe this needs some adjustments.