Make Windows-1250 encoding consistent with other

In sake of consistency with other Windows CP encodings:
* print_name is expanded to "Windows Central European (CP 1250)";
* B_MS_WINDOWS_1250_CONVERSION id looks like should be added into UTF8.h;
* mime_name set to NULL as other windows codepages have. That prevents
  at least from duplicating too much 1250's in the Terminal, Mail and
  StyledEdit encodings menus.
Improve File Encoding StatusView cell. (Fixes #9653)

* Encoding cell of the StyledEdit StatusView is visible now only in case
  the currently opened file encoding is not equal to default UTF-8 one;
* The Encodings menu that was opened by click on this cell is removed;
* Cmd-Opt-PgDn/PgUp shortcuts are added for quick iteration through the
  list of encodings.
Fix "Error opening terminal:xterm-256color" issue (#9636)

Looks like switching to declare "xterm-256color" terminal emulation was
made a bit early: there are lot of servers that still do not know about
this terminal. As was discussed in #9636 the only acceptable way is to
switch back to "xterm" and adjust corresponding entry in our local
termcap database to support 256 colors. So this changeset:

* Declare emulated terminal as "xterm";
* Change the colors and color pairs of "xterm" termcap entry to support
  256 colors;

Workarounds the #9636. Should be upgraded to "xterm-256color" some time
in the future.