Fixed: text cursor for CPTextField class doesn't take content-inset in account

Previously, the text cursor of the CPTextField didn't take in account the content-inset. This raised issue with class as the CPComboBox, when the mouse was over the popUpButton, there was still the text cursor instead of the default one.

Now it works as in Cocoa.
Merge pull request #2250 from Dogild/CursorContentInset

Fixed: text cursor for CPTextField class doesn't take content-inset in account