Temporarily try to find tabwidget images

Long-term plan is to get rid of _u suffixes and rename the files proper.
Remove `position` attribute from tab widget

Use position_technique carefully crafted to be pixel-identical to old
placement code. Needs to move the show() earlier for position_technique
to be parsed: Tabs are drawn relative to the button positions, which are
not yet set before calling show().
Support ImageButton `path` in tabwidget

Removes explicit 'content/gui/' prefix and '_%s.png' suffix.
This makes the tabinterface a bit more friendly for users.

Our diplomacy tab could need a bit of special care since it does weird
things to the image attributes, but this is good enough for now.