Add virtual keyboard support

[khyryra]

Changes

• Add configuration option supportsVirtualKeyboard to libraryexternals.cpp
• If supportsVirtualKeyboard is true, add a TextArea to the AppLoad title bar that captures key events.
• Pass captured key events to the shim, which maps them to X keysyms and sends them to the application. Key press durations and Shift presses are emulated for compatibility. Accent marks are supported.
• Shim creates and writes events to /dev/input/virtual_keyboard.
• If supportsVirtualKeyboard isn't defined or is false, the changes have no effect to the end user.
• No breaking changes.

[khyryra]

I realized I only mapped the first 256 characters in the shim, so the shim will only output ASCII to the app. I'll add Unicode when I get home. Qtfb, isn't affected, it will send all characters correctly over the socket.