Fixes for the TextInput component

[Yatekii]

I wasn't able to fix the vertical layout yet. Something with the height estimator is very off. I guess we need to discuss some more before I can fix it. Also there is too much mixing between PT and PX I feel :)

[coveralls]

Pull Request Test Coverage Report for Build 442

• 0 of 3 (0.0%) changed or added relevant lines in 1 file are covered.
• 12 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-2.0e**-05%**) to 0.048%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/text_layout.rs	0	3	0.0%

Files with Coverage Reduction	New Missed Lines	%
src/window_state.rs	1	0.0%
src/text_layout.rs	11	0.0%

Totals
Change from base Build 425:	-2.0e-05%
Covered Lines:	1
Relevant Lines:	2066

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💛 - Coveralls

[fschutt]

Three small things:

• I would use an Option<VirtualKeyCode> - what if someone presses a non-character key. Sure, for the text component only the char is important, but maybe for other components it's better to match the virtual key, maybe components want to react to non-character keys, too. For example, the text input should later ignore return keys (as it's just one line of text) - it's easier to match on VirtualKeyCode::Return rather than \n.
• I would write it as:

if let Some(key) = keyboard_state.latest_key.and_then(virtual_key_code_to_char) {
    data.text.push(if keyboard_state.shift_down { key.to_uppercase().next().unwrap() } else { key });
}

• When the window goes out of focus, I'd clear the latest_key field like here: `https://github.com/maps4print/azul/blob/e8fd2e15dd104514349833d4a14ed26a0ef742f7/src/window_state.rs#L342-L346