X11: Check for XCreateWindow failure

Check if XCreateWindow fails and bail out.
Add character input events

Add a new event type for unicode text input.

Handle text input in glwt_events test and print out the typed characters
in the window title bar.
X11: Character events using X Input Methods (XIM)

Generate character events from key presses.

Create an X Input Method (XIM) in glwtInit and an X Input Context (XIC)
for each window in glwtWindowCreate.

Use Xutf8LookupString to look up characters on KeyPress events.
Win32: Character events (WM_CHAR)

Handle WM_CHAR messages, decode unicode codepoint from UTF-16.
OSX: Character input events

Process [event characters] in keyDown event handler.