Various parser and UI improvements

Possible item requirements in ItemCompoundParser now limited to items
actually useable as such in-game, i.e. non-equipment items; fixes in
DictionaryStringConverter regarding empty strings; removed saving and
restoring of splitter positions in TableEntryEditor and MessageEditor,
was very unreliable with resized windows; new PropertyGridEx derived
from PropertyGrid, which allows for auto-sizing of columns; other
changes to try and reduce UI flickering when ex. scrolling the
TableEntryEditor's TreeView or selecting elements to be displayed in the
PropertyGrid; other minor changes