GUI: Make text selectable in input boxes

[hax0kartik]

This adds the ability to select text in a EditTextWidget.
The following list of behaviour is implemented:-

• Double Click: Selects all the text
• SHIFT + L/R: Increments/Decrements selection.
• SHIFT + Left Click: Selects text between caret position and the mouse click.
• SHIFT + Home/End : Selects Text between the caretPosition and starting of the string/the end of the string.
• Dragging the mouse over the string will select that part of the string.
• Pressing backspace while a selection has been made will delete that selection.
• Pressing CTRL+C while a selection has been made will copy the selection to clipboard.
• Pressing CTRL+V while a selection has been made will replace the selected text with the text from clipboard.

Many thanks to @sev- for answering my questions and writing the rendering code!

[sev-]

Thank you!