X3270/APL support

Description
x3270 supports both display and input of APL characters.

Display
The 3270 font includes a full set of APL characters; the other 3270 fonts have subsets (mostly the line-drawing characters from code page 310). Many iso10646-1 (Unicode) fonts have the most or all of the APL characters, except for the underlined alphabetics, which are not standard Unicode characters.

Input
In the default keymap, the key combination Shift-Escape performs the following functions: In the temporary keymap, Shift-Escape is changed to undo these three functions.
 * Enables APL mode for paste operations.
 * Sets a temporary keymap to allow APL characters to be entered.
 * Sets a temporary compose map to allow APL characters to be entered.

APL temporary compose map
Symbols marked Sharp come from Sharp APL and are not on code page 310.