implemented a VERY BASIC input control

[tjyuyao]

Demo file can be found in examples/src/input, tested only on Ubuntu. It demonstrate:

• Very simple and basic implementation of a single line of input entry.
• When clicked on any of the input entries, it is selected and you can use the keyboard to type words into it.
• Backspace is implemented to delete the rightmost character.
• Click outside of any input entry will give the control back to other functionality depend on cv::waitKey()
• No cursor control is considered, only editable from the end of string.
• No bleeding is considered, if the text is too long, it will exceed the border.

It is the first time I send a pull request, hope I did it the right way. I will be happy to get feedback on this.

[Dovyski]

This is awesome, thank you! I'll play with this for a while before merging the PR. This is a long-awaited feature, thanks for putting the time to bringing this to life!

[Dovyski]

Ok, merged. I am truly sorry for the absence regarding the maintenance of this project. I am doing my best to allocate time for it.