Handle "&" in exactly the same way as "&" in wxMarkupParser, i.e. do not
map the former to "&&" to prevent it from being interpreted as a mnemonic as
this is incompatible with using markup for anything but the control labels,
e.g. for wxDataViewCtrl items text, in which mnemonics are not recognized.
And even when using markup for control labels, it was a questionable decision
as it's really not clear at all why should the XML entity and the raw
character itself be handled differently.
Also split wxMarkupText into two classes, wxMarkupText that handles
mnemonics in the markup (which is typically a label) and a very
similar, but not derived, wxItemMarkupText that handles mnemonics-less
markup for list etc. items, uses DrawItemText() and supports
ellipsizing.
Illustrate the use of ampersands in the dataview sample.
60bd6842e4