This commit adds and uses wibox.layout.base.fit_widget(). This function is a
wrapper for widget:fit() that caches the result and thus speeds things up.
This is necessary because some layouts call :fit() from their :fit() and :draw()
functions. Nesting such layouts means that at the widget at the tail of the
stack gets its :fit() function called quite often. If this function is not
blazingly fast, this results in noticeable slowness.
Signed-off-by: Uli Schlachter <psychon@znc.in>
3edd216560