popover: Correctly substract arrow height when measuring popover widget
For default popover arrow position and default height-for-width layout mode, natural_width is calculated first with for_size=-1 and orientation=HORIZONTAL, at the end of gtk_popover_measure() natural_width won't be added with tail_height.
Then to measure with for_size=natural_width and orientation=VERTICAL, obviously for_size shouldn't be substract with tail_height.
The wrong logic will force content in popover gets less width and then text labels in popover may get wrapped unnecessarily.
Edited by Yubao Liu