Fixed bugs in, and added operators and copy constructors to, style

Fixed bugs in, and added operators and copy constructors to, style
sheet classes.
Now merges base, paragraph and content styles dynamically on
display or layout so that we can distinguish between them. This
means appling a paragraph style that has character formatting
doesn't wipe out the original content formatting. The old-style
static method can be enabled by setting wxRICHTEXT_USE_DYNAMIC_STYLES
to 0 in richtextbuffer.h.
The default style is initially empty, so that adding text
reflects the base and paragraph styles. Setting the default style
doesn't merge with the previous setting, but BeginStyle does.
Adding new paragraphs is now sensitive to the previous paragraph's
style and also whether its style has a next-paragraph style indication.
Added wxRichTextCtrl::ApplyStyleSheet which reapplies styles to
paragraphs with named styles.
Added style application command to sample to demonstrate how
styles can be changed and reapplied without affecting content
character formatting.
Fixed some range bugs in UI feedback and clipboard copying and pasting.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@41501 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775