Inconsistent behaviour of TextEdit and TextBox widgets under conrod_piston vs. other backends

[lubosmudrak]

Under conrod_piston backend, some actions are made only once and do not continue while certain key is still pressed

Here is a short list:
1.) Holding the backspace key
2.) holding arrow keys to navigate within the text
3.) holding Enter key to add new lines into the text(this issue is present only for TextEdit widget)

I exposed this issue in following pull request: #1432