RFR: 8245053: Keyboard doesn't show when TextInputControl has focus

[Abhinay Agarwal]

In Android, TextInputControls (TextField and TextArea) are responsible for showing and hiding software keyboard.
Currently, a focus listener is attached to these controls and is used to toggle the visibility of keyboard. This
condition fails in cases where the control already has focus but the keyboard is not visible.

Ideally, the keyboard should be shown again when the user taps on the TextInputControl.

This PR adds an event handler for `MouseEvent.MOUSE_CLICKED` event and shows the keyboard  if the TextInput control is
both editable and focused.

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Commit messages:
 - Show keyboard when user taps on TextInput controls

Changes: https://git.openjdk.java.net/jfx/pull/219/files
 Webrev: https://webrevs.openjdk.java.net/jfx/219/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8245053
  Stats: 32 lines in 2 files changed: 6 ins; 6 del; 20 mod
  Patch: https://git.openjdk.java.net/jfx/pull/219.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/219/head:pull/219

PR: https://git.openjdk.java.net/jfx/pull/219