annotation panel height isn't set to minimise white space when view opened/resized

JAL-1691 isn't quite usable yet because the logic for setting the annotation panel height is not working very well when a splitframe is opened, or when an alignment panel is resized.

This looks like a knockon fron JAL-1211 - where Jalview sets the annotation panel height to maximise annotation visibility, whilst ensuring the sequence panel is still visible.

Observed symptoms:
* translating two sequences to protein (whilst testing JAL-1633) results in a splitframe view where no sequences are visible, only the annotation panels - which are maximised to take up the whole panel. If the Desktop pane is larger then the splitframe created is also larger and the sequences are visible, but the default window settings are much smaller so the buggy behaviour is always observed.
* resizing an alignFrame to make it shorter only results in the seqPanel display area being reduced - the annotation panel height stays the same.
 
Originally, I thougt this might be something to do with the way JSplitPane is setting the height of each component - by default the annotation panel is set at 50%, but since each split panel is 50% of the alignFrame, the actual panel height should be 25% - but in fact, it remains at 50% of the parent JFrame height (which includes the JSplitPane). However, the resize behaviour suggests that the logic implemented for JAL-1211 is not allowing the annotation panel/seqpanel split ratio to be modified when a resize event occurs, and is also not calculating the annotation panel height correctly when the alignment is first opened.